diff --git a/sshsig.c b/sshsig.c
new file mode 100644
index 0000000..0a1e146
--- /dev/null
+++ b/sshsig.c
@@ -0,0 +1,787 @@
+/*
+ * Copyright (c) 2019 Google LLC
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <errno.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "authfd.h"
+#include "authfile.h"
+#include "log.h"
+#include "misc.h"
+#include "sshbuf.h"
+#include "sshsig.h"
+#include "ssherr.h"
+#include "sshkey.h"
+#include "match.h"
+#include "digest.h"
+
+#define SIG_VERSION		0x01
+#define MAGIC_PREAMBLE		"SSHSIG"
+#define MAGIC_PREAMBLE_LEN	(sizeof(MAGIC_PREAMBLE) - 1)
+#define BEGIN_SIGNATURE		"-----BEGIN SSH SIGNATURE-----\n"
+#define END_SIGNATURE		"-----END SSH SIGNATURE-----"
+#define RSA_SIGN_ALG		"rsa-sha2-512" /* XXX maybe make configurable */
+#define RSA_SIGN_ALLOWED	"rsa-sha2-512,rsa-sha2-256"
+#define HASHALG_DEFAULT		"sha512" /* XXX maybe make configurable */
+#define HASHALG_ALLOWED		"sha256,sha512"
+
+int
+sshsig_armor(const struct sshbuf *blob, struct sshbuf **out)
+{
+	struct sshbuf *buf = NULL;
+	int r = SSH_ERR_INTERNAL_ERROR;
+
+	*out = NULL;
+
+	if ((buf = sshbuf_new()) == NULL) {
+		error("%s: sshbuf_new failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+
+	if ((r = sshbuf_put(buf, BEGIN_SIGNATURE,
+	    sizeof(BEGIN_SIGNATURE)-1)) != 0) {
+		error("%s: sshbuf_putf failed: %s", __func__, ssh_err(r));
+		goto out;
+	}
+
+	if ((r = sshbuf_dtob64(blob, buf, 1)) != 0) {
+		error("%s: Couldn't base64 encode signature blob: %s",
+		    __func__, ssh_err(r));
+		goto out;
+	}
+
+	if ((r = sshbuf_put(buf, END_SIGNATURE,
+	    sizeof(END_SIGNATURE)-1)) != 0 ||
+	    (r = sshbuf_put_u8(buf, '\n')) != 0) {
+		error("%s: sshbuf_put failed: %s", __func__, ssh_err(r));
+		goto out;
+	}
+	/* success */
+	*out = buf;
+	buf = NULL; /* transferred */
+	r = 0;
+ out:
+	sshbuf_free(buf);
+	return r;
+}
+
+int
+sshsig_dearmor(struct sshbuf *sig, struct sshbuf **out)
+{
+	int r;
+	size_t eoffset = 0;
+	struct sshbuf *buf = NULL;
+	struct sshbuf *sbuf = NULL;
+	char *b64 = NULL;
+
+	if ((sbuf = sshbuf_fromb(sig)) == NULL) {
+		error("%s: sshbuf_fromb failed", __func__);
+		return SSH_ERR_ALLOC_FAIL;
+	}
+
+	if ((r = sshbuf_cmp(sbuf, 0,
+	    BEGIN_SIGNATURE, sizeof(BEGIN_SIGNATURE)-1)) != 0) {
+		error("Couldn't parse signature: missing header");
+		goto done;
+	}
+
+	if ((r = sshbuf_consume(sbuf, sizeof(BEGIN_SIGNATURE)-1)) != 0) {
+		error("%s: sshbuf_consume failed: %s", __func__, ssh_err(r));
+		goto done;
+	}
+
+	if ((r = sshbuf_find(sbuf, 0, "\n" END_SIGNATURE,
+	    sizeof("\n" END_SIGNATURE)-1, &eoffset)) != 0) {
+		error("Couldn't parse signature: missing footer");
+		goto done;
+	}
+
+	if ((r = sshbuf_consume_end(sbuf, sshbuf_len(sbuf)-eoffset)) != 0) {
+		error("%s: sshbuf_consume failed: %s", __func__, ssh_err(r));
+		goto done;
+	}
+
+	if ((b64 = sshbuf_dup_string(sbuf)) == NULL) {
+		error("%s: sshbuf_dup_string failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto done;
+	}
+
+	if ((buf = sshbuf_new()) == NULL) {
+		error("%s: sshbuf_new() failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto done;
+	}
+
+	if ((r = sshbuf_b64tod(buf, b64)) != 0) {
+		error("Coundn't decode signature: %s", ssh_err(r));
+		goto done;
+	}
+
+	/* success */
+	*out = buf;
+	r = 0;
+	buf = NULL; /* transferred */
+done:
+	sshbuf_free(buf);
+	sshbuf_free(sbuf);
+	free(b64);
+	return r;
+}
+
+static int
+sshsig_wrap_sign(struct sshkey *key, const char *hashalg,
+    const struct sshbuf *h_message, const char *sig_namespace,
+    struct sshbuf **out, sshsig_signer *signer, void *signer_ctx)
+{
+	int r;
+	size_t slen = 0;
+	u_char *sig = NULL;
+	struct sshbuf *blob = NULL;
+	struct sshbuf *tosign = NULL;
+	const char *sign_alg = NULL;
+
+	if ((tosign = sshbuf_new()) == NULL ||
+	    (blob = sshbuf_new()) == NULL) {
+		error("%s: sshbuf_new failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto done;
+	}
+
+	if ((r = sshbuf_put(tosign, MAGIC_PREAMBLE, MAGIC_PREAMBLE_LEN)) != 0 ||
+	    (r = sshbuf_put_cstring(tosign, sig_namespace)) != 0 ||
+	    (r = sshbuf_put_string(tosign, NULL, 0)) != 0 || /* reserved */
+	    (r = sshbuf_put_cstring(tosign, hashalg)) != 0 ||
+	    (r = sshbuf_putb(tosign, h_message)) != 0) {
+		error("Couldn't construct message to sign: %s", ssh_err(r));
+		goto done;
+	}
+
+	/* If using RSA keys then default to a good signature algorithm */
+	if (sshkey_type_plain(key->type) == KEY_RSA)
+		sign_alg = RSA_SIGN_ALG;
+
+	if (signer != NULL) {
+		if ((r = signer(key, &sig, &slen,
+		    sshbuf_ptr(tosign), sshbuf_len(tosign),
+		    sign_alg, 0, signer_ctx)) != 0) {
+			error("Couldn't sign message: %s", ssh_err(r));
+			goto done;
+		}
+	} else {
+		if ((r = sshkey_sign(key, &sig, &slen,
+		    sshbuf_ptr(tosign), sshbuf_len(tosign),
+		    sign_alg, 0)) != 0) {
+			error("Couldn't sign message: %s", ssh_err(r));
+			goto done;
+		}
+	}
+
+	if ((r = sshbuf_put(blob, MAGIC_PREAMBLE, MAGIC_PREAMBLE_LEN)) != 0 ||
+	    (r = sshbuf_put_u32(blob, SIG_VERSION)) != 0 ||
+	    (r = sshkey_puts(key, blob)) != 0 ||
+	    (r = sshbuf_put_cstring(blob, sig_namespace)) != 0 ||
+	    (r = sshbuf_put_string(blob, NULL, 0)) != 0 || /* reserved */
+	    (r = sshbuf_put_cstring(blob, hashalg)) != 0 ||
+	    (r = sshbuf_put_string(blob, sig, slen)) != 0) {
+		error("Couldn't populate blob: %s", ssh_err(r));
+		goto done;
+	}
+
+	*out = blob;
+	blob = NULL;
+	r = 0;
+done:
+	free(sig);
+	sshbuf_free(blob);
+	sshbuf_free(tosign);
+	return r;
+}
+
+/* Check preamble and version. */
+static int
+sshsig_parse_preamble(struct sshbuf *buf)
+{
+	int r = SSH_ERR_INTERNAL_ERROR;
+	uint32_t sversion;
+
+	if ((r = sshbuf_cmp(buf, 0, MAGIC_PREAMBLE, MAGIC_PREAMBLE_LEN)) != 0 ||
+	    (r = sshbuf_consume(buf, (sizeof(MAGIC_PREAMBLE)-1))) != 0 ||
+	    (r = sshbuf_get_u32(buf, &sversion)) != 0) {
+		error("Couldn't verify signature: invalid format");
+		return r;
+	}
+
+	if (sversion < SIG_VERSION) {
+		error("Signature version %lu is larger than supported "
+		    "version %u", (unsigned long)sversion, SIG_VERSION);
+		return SSH_ERR_INVALID_FORMAT;
+	}
+	return 0;
+}
+
+static int
+sshsig_check_hashalg(const char *hashalg)
+{
+	if (match_pattern_list(hashalg, HASHALG_ALLOWED, 0) == 1)
+		return 0;
+	error("%s: unsupported hash algorithm \"%.100s\"", __func__, hashalg);
+	return SSH_ERR_SIGN_ALG_UNSUPPORTED;
+}
+
+static int
+sshsig_peek_hashalg(struct sshbuf *signature, char **hashalgp)
+{
+	struct sshbuf *buf = NULL;
+	char *hashalg = NULL;
+	int r = SSH_ERR_INTERNAL_ERROR;
+
+	if (hashalgp != NULL)
+		*hashalgp = NULL;
+	if ((buf = sshbuf_fromb(signature)) == NULL)
+		return SSH_ERR_ALLOC_FAIL;
+	if ((r = sshsig_parse_preamble(buf)) != 0)
+		goto done;
+	if ((r = sshbuf_get_string_direct(buf, NULL, NULL)) != 0 ||
+	    (r = sshbuf_get_string_direct(buf, NULL, NULL)) != 0 ||
+	    (r = sshbuf_get_string(buf, NULL, NULL)) != 0 ||
+	    (r = sshbuf_get_cstring(buf, &hashalg, NULL)) != 0 ||
+	    (r = sshbuf_get_string_direct(buf, NULL, NULL)) != 0) {
+		error("Couldn't parse signature blob: %s", ssh_err(r));
+		goto done;
+	}
+	if ((r = sshsig_check_hashalg(hashalg)) != 0)
+		goto done;
+
+	/* success */
+	r = 0;
+	*hashalgp = hashalg;
+	hashalg = NULL;
+ done:
+	free(hashalg);
+	sshbuf_free(buf);
+	return r;
+}
+
+static int
+sshsig_wrap_verify(struct sshbuf *signature, const char *hashalg,
+    const struct sshbuf *h_message, const char *expect_namespace,
+    struct sshkey **sign_keyp)
+{
+	int r = SSH_ERR_INTERNAL_ERROR;
+	struct sshbuf *buf = NULL, *toverify = NULL;
+	struct sshkey *key = NULL;
+	const u_char *sig;
+	char *got_namespace = NULL, *sigtype = NULL, *sig_hashalg = NULL;
+	size_t siglen;
+
+	if (sign_keyp != NULL)
+		*sign_keyp = NULL;
+
+	if ((toverify = sshbuf_new()) == NULL) {
+		error("%s: sshbuf_new failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto done;
+	}
+	if ((r = sshsig_check_hashalg(hashalg)) != 0)
+		goto done;
+
+	if ((r = sshbuf_put(toverify, MAGIC_PREAMBLE,
+	    MAGIC_PREAMBLE_LEN)) != 0 ||
+	    (r = sshbuf_put_cstring(toverify, expect_namespace)) != 0 ||
+	    (r = sshbuf_put_string(toverify, NULL, 0)) != 0 || /* reserved */
+	    (r = sshbuf_put_cstring(toverify, hashalg)) != 0 ||
+	    (r = sshbuf_putb(toverify, h_message)) != 0) {
+		error("Couldn't construct message to verify: %s", ssh_err(r));
+		goto done;
+	}
+
+	if ((r = sshsig_parse_preamble(signature)) != 0)
+		goto done;
+
+	if ((r = sshkey_froms(signature, &key)) != 0 ||
+	    (r = sshbuf_get_cstring(signature, &got_namespace, NULL)) != 0 ||
+	    (r = sshbuf_get_string(signature, NULL, NULL)) != 0 ||
+	    (r = sshbuf_get_cstring(signature, &sig_hashalg, NULL)) != 0 ||
+	    (r = sshbuf_get_string_direct(signature, &sig, &siglen)) != 0) {
+		error("Couldn't parse signature blob: %s", ssh_err(r));
+		goto done;
+	}
+
+	if (sshbuf_len(signature) != 0) {
+		error("Signature contains trailing data");
+		r = SSH_ERR_INVALID_FORMAT;
+		goto done;
+	}
+
+	if (strcmp(expect_namespace, got_namespace) != 0) {
+		error("Couldn't verify signature: namespace does not match");
+		debug("%s: expected namespace \"%s\" received \"%s\"",
+		    __func__, expect_namespace, got_namespace);
+		r = SSH_ERR_SIGNATURE_INVALID;
+		goto done;
+	}
+	if (strcmp(hashalg, sig_hashalg) != 0) {
+		error("Couldn't verify signature: hash algorithm mismatch");
+		debug("%s: expected algorithm \"%s\" received \"%s\"",
+		    __func__, hashalg, sig_hashalg);
+		r = SSH_ERR_SIGNATURE_INVALID;
+		goto done;
+	}
+	/* Ensure that RSA keys use an acceptable signature algorithm */
+	if (sshkey_type_plain(key->type) == KEY_RSA) {
+		if ((r = sshkey_get_sigtype(sig, siglen, &sigtype)) != 0) {
+			error("Couldn't verify signature: unable to get "
+			    "signature type: %s", ssh_err(r));
+			goto done;
+		}
+		if (match_pattern_list(sigtype, RSA_SIGN_ALLOWED, 0) != 1) {
+			error("Couldn't verify signature: unsupported RSA "
+			    "signature algorithm %s", sigtype);
+			r = SSH_ERR_SIGN_ALG_UNSUPPORTED;
+			goto done;
+		}
+	}
+	if ((r = sshkey_verify(key, sig, siglen, sshbuf_ptr(toverify),
+	    sshbuf_len(toverify), NULL, 0)) != 0) {
+		error("Signature verification failed: %s", ssh_err(r));
+		goto done;
+	}
+
+	/* success */
+	r = 0;
+	if (sign_keyp != NULL) {
+		*sign_keyp = key;
+		key = NULL; /* transferred */
+	}
+done:
+	free(got_namespace);
+	free(sigtype);
+	free(sig_hashalg);
+	sshbuf_free(buf);
+	sshbuf_free(toverify);
+	sshkey_free(key);
+	return r;
+}
+
+int
+sshsig_sign_message(struct sshkey *key, const char *hashalg,
+    const struct sshbuf *message, const char *sig_namespace,
+    struct sshbuf **out, sshsig_signer *signer, void *signer_ctx)
+{
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	struct sshbuf *b = NULL;
+	int alg, r = SSH_ERR_INTERNAL_ERROR;
+
+	if (out != NULL)
+		*out = NULL;
+	if (hashalg == NULL)
+	    hashalg = HASHALG_DEFAULT;
+
+	if ((r = sshsig_check_hashalg(hashalg)) != 0)
+		return r;
+	if ((alg = ssh_digest_alg_by_name(hashalg)) == -1) {
+		error("%s: can't look up hash algorithm %s",
+		    __func__, HASHALG_DEFAULT);
+		return SSH_ERR_INTERNAL_ERROR;
+	}
+	if ((r = ssh_digest_buffer(alg, message, hash, sizeof(hash))) != 0) {
+		error("%s: ssh_digest_buffer failed: %s", __func__, ssh_err(r));
+		return r;
+	}
+	if ((b = sshbuf_from(hash, ssh_digest_bytes(alg))) == NULL) {
+		error("%s: sshbuf_from failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshsig_wrap_sign(key, hashalg, b, sig_namespace, out,
+	    signer, signer_ctx)) != 0)
+		goto out;
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(b);
+	explicit_bzero(hash, sizeof(hash));
+	return r;
+}
+
+int
+sshsig_verify_message(struct sshbuf *signature, const struct sshbuf *message,
+    const char *expect_namespace, struct sshkey **sign_keyp)
+{
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	struct sshbuf *b = NULL;
+	int alg, r = SSH_ERR_INTERNAL_ERROR;
+	char *hashalg = NULL;
+
+	if (sign_keyp != NULL)
+		*sign_keyp = NULL;
+
+	if ((r = sshsig_peek_hashalg(signature, &hashalg)) != 0)
+		return r;
+	if ((alg = ssh_digest_alg_by_name(hashalg)) == -1) {
+		error("%s: can't look up hash algorithm %s",
+		    __func__, HASHALG_DEFAULT);
+		return SSH_ERR_INTERNAL_ERROR;
+	}
+	if ((r = ssh_digest_buffer(alg, message, hash, sizeof(hash))) != 0) {
+		error("%s: ssh_digest_buffer failed: %s", __func__, ssh_err(r));
+		goto out;
+	}
+	if ((b = sshbuf_from(hash, ssh_digest_bytes(alg))) == NULL) {
+		error("%s: sshbuf_from failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshsig_wrap_verify(signature, hashalg, b, expect_namespace,
+	    sign_keyp)) != 0)
+		goto out;
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(b);
+	free(hashalg);
+	explicit_bzero(hash, sizeof(hash));
+	return r;
+}
+
+static int
+hash_file(int fd, int hashalg, u_char *hash, size_t hashlen)
+{
+	char *hex, rbuf[8192];
+	ssize_t n, total = 0;
+	struct ssh_digest_ctx *ctx;
+	int r, oerrno;
+
+	memset(hash, 0, hashlen);
+	if ((ctx = ssh_digest_start(hashalg)) == NULL) {
+		error("%s: ssh_digest_start failed", __func__);
+		return SSH_ERR_INTERNAL_ERROR;
+	}
+	for (;;) {
+		if ((n = read(fd, rbuf, sizeof(rbuf))) == -1) {
+			if (errno == EINTR || errno == EAGAIN)
+				continue;
+			oerrno = errno;
+			error("%s: read: %s", __func__, strerror(errno));
+			ssh_digest_free(ctx);
+			errno = oerrno;
+			return SSH_ERR_SYSTEM_ERROR;
+		} else if (n == 0) {
+			debug2("%s: hashed %zu bytes", __func__, total);
+			break; /* EOF */
+		}
+		total += (size_t)n;
+		if ((r = ssh_digest_update(ctx, rbuf, (size_t)n)) != 0) {
+			error("%s: ssh_digest_update: %s",
+			    __func__, ssh_err(r));
+			ssh_digest_free(ctx);
+			return r;
+		}
+	}
+	if ((r = ssh_digest_final(ctx, hash, hashlen)) != 0) {
+		error("%s: ssh_digest_final: %s", __func__, ssh_err(r));
+		ssh_digest_free(ctx);
+	}
+	if ((hex = tohex(hash, hashlen)) != NULL) {
+		debug3("%s: final hash: %s", __func__, hex);
+		freezero(hex, strlen(hex));
+	}
+	/* success */
+	ssh_digest_free(ctx);
+	return 0;
+}
+
+int
+sshsig_sign_fd(struct sshkey *key, const char *hashalg,
+    int fd, const char *sig_namespace, struct sshbuf **out,
+    sshsig_signer *signer, void *signer_ctx)
+{
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	struct sshbuf *b = NULL;
+	int alg, r = SSH_ERR_INTERNAL_ERROR;
+
+	if (out != NULL)
+		*out = NULL;
+	if (hashalg == NULL)
+	    hashalg = HASHALG_DEFAULT;
+
+	if ((r = sshsig_check_hashalg(hashalg)) != 0)
+		return r;
+	if ((alg = ssh_digest_alg_by_name(hashalg)) == -1) {
+		error("%s: can't look up hash algorithm %s",
+		    __func__, HASHALG_DEFAULT);
+		return SSH_ERR_INTERNAL_ERROR;
+	}
+	if ((r = hash_file(fd, alg, hash, sizeof(hash))) != 0) {
+		error("%s: hash_file failed: %s", __func__, ssh_err(r));
+		return r;
+	}
+	if ((b = sshbuf_from(hash, ssh_digest_bytes(alg))) == NULL) {
+		error("%s: sshbuf_from failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshsig_wrap_sign(key, hashalg, b, sig_namespace, out,
+	    signer, signer_ctx)) != 0)
+		goto out;
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(b);
+	explicit_bzero(hash, sizeof(hash));
+	return r;
+}
+
+int
+sshsig_verify_fd(struct sshbuf *signature, int fd,
+    const char *expect_namespace, struct sshkey **sign_keyp)
+{
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	struct sshbuf *b = NULL;
+	int alg, r = SSH_ERR_INTERNAL_ERROR;
+	char *hashalg = NULL;
+
+	if (sign_keyp != NULL)
+		*sign_keyp = NULL;
+
+	if ((r = sshsig_peek_hashalg(signature, &hashalg)) != 0)
+		return r;
+	if ((alg = ssh_digest_alg_by_name(hashalg)) == -1) {
+		error("%s: can't look up hash algorithm %s",
+		    __func__, HASHALG_DEFAULT);
+		return SSH_ERR_INTERNAL_ERROR;
+	}
+	if ((r = hash_file(fd, alg, hash, sizeof(hash))) != 0) {
+		error("%s: hash_file failed: %s", __func__, ssh_err(r));
+		return r;
+	}
+	if ((b = sshbuf_from(hash, ssh_digest_bytes(alg))) == NULL) {
+		error("%s: sshbuf_from failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshsig_wrap_verify(signature, hashalg, b, expect_namespace,
+	    sign_keyp)) != 0)
+		goto out;
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(b);
+	free(hashalg);
+	explicit_bzero(hash, sizeof(hash));
+	return r;
+}
+
+struct sigopts {
+	int ca;
+	char *namespaces;
+};
+
+static struct sigopts *
+sigopts_parse(const char *opts, const char *path, u_long linenum,
+    const char **errstrp)
+{
+	struct sigopts *ret;
+	int r;
+	const char *errstr = NULL;
+
+	if ((ret = calloc(1, sizeof(*ret))) == NULL)
+		return NULL;
+	if (opts == NULL || *opts == '\0')
+		return ret; /* Empty options yields empty options :) */
+
+	while (*opts && *opts != ' ' && *opts != '\t') {
+		/* flag options */
+		if ((r = opt_flag("cert-authority", 0, &opts)) != -1) {
+			ret->ca = 1;
+		} else if (opt_match(&opts, "namespaces")) {
+			if (ret->namespaces != NULL) {
+				errstr = "multiple \"namespaces\" clauses";
+				goto fail;
+			}
+			ret->namespaces = opt_dequote(&opts, &errstr);
+			if (ret->namespaces == NULL)
+				goto fail;
+		}
+		/*
+		 * Skip the comma, and move to the next option
+		 * (or break out if there are no more).
+		 */
+		if (*opts == '\0' || *opts == ' ' || *opts == '\t')
+			break;		/* End of options. */
+		/* Anything other than a comma is an unknown option */
+		if (*opts != ',') {
+			errstr = "unknown key option";
+			goto fail;
+		}
+		opts++;
+		if (*opts == '\0') {
+			errstr = "unexpected end-of-options";
+			goto fail;
+		}
+	}
+	/* success */
+	return ret;
+ fail:
+	if (errstrp != NULL)
+		*errstrp = errstr;
+	free(ret);
+	return NULL;
+}
+
+static void
+sigopts_free(struct sigopts *opts)
+{
+	if (opts == NULL)
+		return;
+	free(opts->namespaces);
+	free(opts);
+}
+
+static int
+check_allowed_keys_line(const char *path, u_long linenum, char *line,
+    const struct sshkey *sign_key, const char *principal,
+    const char *sig_namespace)
+{
+	struct sshkey *found_key = NULL;
+	char *cp, *opts = NULL, *identities = NULL;
+	int r, found = 0;
+	const char *reason = NULL;
+	struct sigopts *sigopts = NULL;
+
+	if ((found_key = sshkey_new(KEY_UNSPEC)) == NULL) {
+		error("%s: sshkey_new failed", __func__);
+		return SSH_ERR_ALLOC_FAIL;
+	}
+
+	/* format: identity[,identity...] [option[,option...]] key */
+	cp = line;
+	cp = cp + strspn(cp, " \t"); /* skip leading whitespace */
+	if (*cp == '#' || *cp == '\0')
+		goto done;
+	if ((identities = strdelimw(&cp)) == NULL) {
+		error("%s:%lu: invalid line", path, linenum);
+		goto done;
+	}
+	if (match_pattern_list(principal, identities, 0) != 1) {
+		/* principal didn't match */
+		goto done;
+	}
+	debug("%s: %s:%lu: matched principal \"%s\"",
+	    __func__, path, linenum, principal);
+
+	if (sshkey_read(found_key, &cp) != 0) {
+		/* no key? Check for options */
+		opts = cp;
+		if (sshkey_advance_past_options(&cp) != 0) {
+			error("%s:%lu: invalid options",
+			    path, linenum);
+			goto done;
+		}
+		*cp++ = '\0';
+		skip_space(&cp);
+		if (sshkey_read(found_key, &cp) != 0) {
+			error("%s:%lu: invalid key", path,
+			    linenum);
+			goto done;
+		}
+	}
+	debug3("%s:%lu: options %s", path, linenum, opts == NULL ? "" : opts);
+	if ((sigopts = sigopts_parse(opts, path, linenum, &reason)) == NULL) {
+		error("%s:%lu: bad options: %s", path, linenum, reason);
+		goto done;
+	}
+
+	/* Check whether options preclude the use of this key */
+	if (sigopts->namespaces != NULL &&
+	    match_pattern_list(sig_namespace, sigopts->namespaces, 0) != 1) {
+		error("%s:%lu: key is not permitted for use in signature "
+		    "namespace \"%s\"", path, linenum, sig_namespace);
+		goto done;
+	}
+
+	if (!sigopts->ca && sshkey_equal(found_key, sign_key)) {
+		/* Exact match of key */
+		debug("%s:%lu: matched key and principal", path, linenum);
+		/* success */
+		found = 1;
+	} else if (sigopts->ca && sshkey_is_cert(sign_key) &&
+	    sshkey_equal_public(sign_key->cert->signature_key, found_key)) {
+		/* Match of certificate's CA key */
+		if ((r = sshkey_cert_check_authority(sign_key, 0, 1,
+		    principal, &reason)) != 0) {
+			error("%s:%lu: certificate not authorized: %s",
+			    path, linenum, reason);
+			goto done;
+		}
+		debug("%s:%lu: matched certificate CA key", path, linenum);
+		/* success */
+		found = 1;
+	} else {
+		/* Principal matched but key didn't */
+		goto done;
+	}
+ done:
+	sshkey_free(found_key);
+	sigopts_free(sigopts);
+	return found ? 0 : SSH_ERR_KEY_NOT_FOUND;
+}
+
+int
+sshsig_check_allowed_keys(const char *path, const struct sshkey *sign_key,
+    const char *principal, const char *sig_namespace)
+{
+	FILE *f = NULL;
+	char *line = NULL;
+	size_t linesize = 0;
+	u_long linenum = 0;
+	int r, oerrno;
+
+	/* Check key and principal against file */
+	if ((f = fopen(path, "r")) == NULL) {
+		oerrno = errno;
+		error("Unable to open allowed keys file \"%s\": %s",
+		    path, strerror(errno));
+		errno = oerrno;
+		return SSH_ERR_SYSTEM_ERROR;
+	}
+
+	while (getline(&line, &linesize, f) != -1) {
+		linenum++;
+		r = check_allowed_keys_line(path, linenum, line, sign_key,
+		    principal, sig_namespace);
+		if (r == SSH_ERR_KEY_NOT_FOUND)
+			continue;
+		else if (r == 0) {
+			/* success */
+			fclose(f);
+			free(line);
+			return 0;
+			/* XXX continue and check revocation? */
+		} else
+			break;
+	}
+	/* Either we hit an error parsing or we simply didn't find the key */
+	fclose(f);
+	free(line);
+	return r == 0 ? SSH_ERR_KEY_NOT_FOUND : r;
+}