On Mon, Jan 05, 2026 at 03:21:31PM +0000, David Howells wrote:
> Add support for RSASSA-PSS [RFC8017 sec 8.1] signature verification support
> to the RSA driver in crypto/. Note that signing support is not provided.
>
> The verification function requires an info string formatted as a
> space-separated list of key=value pairs. The following parameters need to
> be provided:
>
> (1) sighash=<algo>
>
> The hash algorithm to be used to digest the data.
>
> (2) pss_mask=<type>,...
>
> The mask generation function (MGF) and its parameters.
>
> (3) pss_salt=<len>
>
> The length of the salt used.
>
> The only MGF currently supported is "mgf1". This takes an additional
> parameter indicating the mask-generating hash (which need not be the same
> as the data hash). E.g.:
>
> "sighash=sha256 pss_mask=mgf1,sha256 pss_salt=32"
>
> Signed-off-by: David Howells <[email protected]>
> cc: Tadeusz Struk <[email protected]>
> cc: Herbert Xu <[email protected]>
> cc: David S. Miller <[email protected]>
> cc: Lukas Wunner <[email protected]>
> cc: Ignat Korchagin <[email protected]>
> cc: [email protected]
> cc: [email protected]
> ---
> crypto/Makefile | 1 +
> crypto/rsa.c | 8 +
> crypto/rsassa-pss.c | 397 ++++++++++++++++++++++++++++++++++
> include/crypto/internal/rsa.h | 2 +
> 4 files changed, 408 insertions(+)
> create mode 100644 crypto/rsassa-pss.c
>
> diff --git a/crypto/Makefile b/crypto/Makefile
> index 267d5403045b..5c91440d1751 100644
> --- a/crypto/Makefile
> +++ b/crypto/Makefile
> @@ -50,6 +50,7 @@ rsa_generic-y += rsa.o
> rsa_generic-y += rsa_helper.o
> rsa_generic-y += rsa-pkcs1pad.o
> rsa_generic-y += rsassa-pkcs1.o
> +rsa_generic-y += rsassa-pss.o
> obj-$(CONFIG_CRYPTO_RSA) += rsa_generic.o
>
> $(obj)/ecdsasignature.asn1.o: $(obj)/ecdsasignature.asn1.c
> $(obj)/ecdsasignature.asn1.h
> diff --git a/crypto/rsa.c b/crypto/rsa.c
> index 6c7734083c98..189a09d54c16 100644
> --- a/crypto/rsa.c
> +++ b/crypto/rsa.c
> @@ -10,6 +10,7 @@
> #include <linux/mpi.h>
> #include <crypto/internal/rsa.h>
> #include <crypto/internal/akcipher.h>
> +#include <crypto/internal/sig.h>
> #include <crypto/akcipher.h>
> #include <crypto/algapi.h>
>
> @@ -414,8 +415,14 @@ static int __init rsa_init(void)
> if (err)
> goto err_unregister_rsa_pkcs1pad;
>
> + err = crypto_register_sig(&rsassa_pss_alg);
> + if (err)
> + goto err_rsassa_pss;
> +
> return 0;
>
> +err_rsassa_pss:
> + crypto_unregister_template(&rsassa_pkcs1_tmpl);
> err_unregister_rsa_pkcs1pad:
> crypto_unregister_template(&rsa_pkcs1pad_tmpl);
> err_unregister_rsa:
> @@ -425,6 +432,7 @@ static int __init rsa_init(void)
>
> static void __exit rsa_exit(void)
> {
> + crypto_unregister_sig(&rsassa_pss_alg);
> crypto_unregister_template(&rsassa_pkcs1_tmpl);
> crypto_unregister_template(&rsa_pkcs1pad_tmpl);
> crypto_unregister_akcipher(&rsa);
> diff --git a/crypto/rsassa-pss.c b/crypto/rsassa-pss.c
> new file mode 100644
> index 000000000000..7f27e8fa6fa7
> --- /dev/null
> +++ b/crypto/rsassa-pss.c
> @@ -0,0 +1,397 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * RSA Signature Scheme combined with EMSA-PSS encoding (RFC 8017 sec 8.2)
> + *
> + * https://www.rfc-editor.org/rfc/rfc8017#section-8.1
> + *
> + * Copyright (c) 2025 Red Hat
> + */
> +
> +#define pr_fmt(fmt) "RSAPSS: "fmt
> +#include <linux/ctype.h>
> +#include <linux/module.h>
> +#include <linux/oid_registry.h>
> +#include <linux/parser.h>
> +#include <linux/scatterlist.h>
> +#include <crypto/akcipher.h>
> +#include <crypto/algapi.h>
> +#include <crypto/hash.h>
> +#include <crypto/sig.h>
> +#include <crypto/internal/akcipher.h>
> +#include <crypto/internal/rsa.h>
> +#include <crypto/internal/sig.h>
> +
> +struct rsassa_pss_ctx {
> + struct crypto_akcipher *rsa;
> + unsigned int key_size;
> + unsigned int salt_len;
> + char *pss_hash;
> + char *mgf1_hash;
> +};
Just a nit but I would not align these fields as it does not serve any
purpose here.
> +
> +enum {
> + rsassa_pss_verify_hash_algo,
> + rsassa_pss_verify_pss_mask,
> + rsassa_pss_verify_pss_salt,
> +};
> +
> +static const match_table_t rsassa_pss_verify_params = {
> + { rsassa_pss_verify_hash_algo, "sighash=%s" },
> + { rsassa_pss_verify_pss_mask, "pss_mask=%s" },
> + { rsassa_pss_verify_pss_salt, "pss_salt=%u" },
> + {}
> +};
> +
> +/*
> + * Parse the signature parameters out of the info string.
> + */
> +static int rsassa_pss_vinfo_parse(struct rsassa_pss_ctx *ctx,
> + char *info)
> +{
> + substring_t args[MAX_OPT_ARGS];
> + char *p, *q;
> +
> + ctx->pss_hash = NULL;
> + ctx->mgf1_hash = NULL;
> + ctx->salt_len = 0;
> +
> + for (p = info; p && *p;) {
> + if (isspace(*p)) {
> + p++;
> + continue;
> + }
> + q = p++;
> + while (*p && !isspace(*p))
> + p++;
> +
> + if (!*p)
> + p = NULL;
> + else
> + *p++ = 0;
> +
> + switch (match_token(q, rsassa_pss_verify_params, args)) {
> + case rsassa_pss_verify_hash_algo:
> + *args[0].to = 0;
> + ctx->pss_hash = args[0].from;
> + break;
> + case rsassa_pss_verify_pss_mask:
> + if (memcmp(args[0].from, "mgf1", 4) != 0)
> + return -ENOPKG;
> + if (args[0].from[4] != ',')
> + return -EINVAL;
> + args[0].from += 5;
> + if (args[0].from >= args[0].to)
> + return -EINVAL;
> + *args[0].to = 0;
> + ctx->mgf1_hash = args[0].from;
> + break;
> + case rsassa_pss_verify_pss_salt:
> + if (match_uint(&args[0], &ctx->salt_len) < 0)
> + return -EINVAL;
> + break;
> + default:
> + pr_debug("Unknown info param\n");
> + return -EINVAL; /* Ignoring it might be better. */
> + }
> + }
> +
> + if (!ctx->pss_hash ||
> + !ctx->mgf1_hash ||
> + !ctx->salt_len)
> + return -EINVAL;
> + return 0;
> +}
> +
> +DEFINE_FREE(crypto_free_shash, struct crypto_shash*,
> + if (!IS_ERR_OR_NULL(_T)) { crypto_free_shash(_T); });
> +
> +/*
> + * Perform mask = MGF1(mgfSeed, masklen) - RFC8017 appendix B.2.1.
> + */
> +static int MGF1(struct rsassa_pss_ctx *ctx,
> + const u8 *mgfSeed, unsigned int mgfSeed_len,
> + u8 *mask, unsigned int maskLen)
> +{
> + struct crypto_shash *hash_tfm __free(crypto_free_shash) = NULL;
> + struct shash_desc *Hash __free(kfree) = NULL;
> + unsigned int counter, count_to, hLen, T_len;
> + __be32 *C;
> + int err;
> + u8 *T, *t, *to_hash;
> +
> + hash_tfm = crypto_alloc_shash(ctx->mgf1_hash, 0, 0);
> + if (IS_ERR(hash_tfm))
> + return PTR_ERR(hash_tfm);
> +
> + hLen = crypto_shash_digestsize(hash_tfm);
> + count_to = DIV_ROUND_UP(maskLen, hLen);
> + T_len = hLen * count_to;
> +
> + Hash = kmalloc(roundup(sizeof(struct shash_desc) +
> + crypto_shash_descsize(hash_tfm), 64) +
> + roundup(T_len, 64) + /* T */
> + roundup(mgfSeed_len + 4, 64), /* mgfSeed||C */
> + GFP_KERNEL);
> + if (!Hash)
> + return -ENOMEM;
> +
> + Hash->tfm = hash_tfm;
> +
> + /* 2: Let T be the empty octet string. */
> + T = (void *)Hash +
> + roundup(sizeof(struct shash_desc) +
> + crypto_shash_descsize(hash_tfm), 64);
> +
> + /* 3: Generate the mask. */
> + to_hash = T + roundup(T_len, 64);
> + memcpy(to_hash, mgfSeed, mgfSeed_len);
> + C = (__be32 *)(to_hash + mgfSeed_len);
> +
> + t = T;
> + for (counter = 0; counter < count_to; counter++) {
> + /* 3A: C = I2OSP(counter, 4). */
> + put_unaligned_be32(counter, C);
> +
> + /* 3B: T = T || Hash(mgfSeed || C). */
> + err = crypto_shash_digest(Hash, to_hash, mgfSeed_len + 4, t);
> + if (err < 0)
> + return err;
> +
> + t += hLen;
> + }
> +
> + /* 4: Output T to mask */
> + memcpy(mask, T, maskLen);
> + return 0;
> +}
> +
> +/*
> + * Perform EMSA-PSS-VERIFY(M, EM, emBits) - RFC8017 sec 9.1.2.
> + */
> +static int emsa_pss_verify(struct rsassa_pss_ctx *ctx,
> + const u8 *M, unsigned int M_len,
> + const u8 *EM, unsigned int emLen)
> +{
> + struct crypto_shash *hash_tfm __free(crypto_free_shash);
> + struct shash_desc *Hash __free(kfree) = NULL;
> + unsigned int emBits, hLen, sLen, DB_len;
> + const u8 *maskedDB, *H;
> + u8 *mHash, *dbMask, *DB, *salt, *Mprime, *Hprime;
> + int err, i;
> +
> + emBits = 8 - fls(EM[0]);
> + emBits = emLen * 8 - emBits;
> +
> + hash_tfm = crypto_alloc_shash(ctx->pss_hash, 0, 0);
> + if (IS_ERR(hash_tfm))
> + return PTR_ERR(hash_tfm);
> +
> + hLen = crypto_shash_digestsize(hash_tfm);
> + sLen = ctx->salt_len;
> +
> + if (sLen > 65536 ||
> + emBits < 8 * (hLen + sLen) + 9)
> + return -EBADMSG;
> +
> + DB_len = emLen - hLen - 1;
> +
> + Hash = kmalloc(roundup(sizeof(struct shash_desc) +
> + crypto_shash_descsize(hash_tfm), 64) +
> + roundup(hLen, 64) + /* mHash */
> + roundup(DB_len, 64) + /* DB and dbMask */
> + roundup(8 + hLen + sLen, 64) + /* M' */
> + roundup(hLen, 64), /* H' */
> + GFP_KERNEL);
> + if (!Hash)
> + return -ENOMEM;
> +
> + Hash->tfm = hash_tfm;
> +
> + mHash = (void *)Hash +
> + roundup(sizeof(struct shash_desc) +
> + crypto_shash_descsize(hash_tfm), 64);
> + DB = dbMask = mHash + roundup(hLen, 64);
> + Mprime = dbMask + roundup(DB_len, 64);
> + Hprime = Mprime + roundup(8 + hLen + sLen, 64);
> +
> + /* 1. Check len M against hash input limitation. */
> + /* The standard says ~2EiB for SHA1, so I think we can ignore this. */
> +
> + /* 2. mHash = Hash(M).
> + * In theory, we would do:
> + * err = crypto_shash_digest(Hash, M, M_len, mHash);
> + * but the caller is assumed to already have done that for us.
> + */
> + if (M_len != hLen)
> + return -EINVAL;
> + memcpy(mHash, M, hLen);
> +
> + /* 3. Check emLen against hLen + sLen + 2. */
> + if (emLen < hLen + sLen + 2)
> + return -EBADMSG;
> +
> + /* 4. Validate EM. */
> + if (EM[emLen - 1] != 0xbc)
> + return -EKEYREJECTED;
> +
> + /* 5. Pick maskedDB and H. */
> + maskedDB = EM;
> + H = EM + DB_len;
> +
> + /* 6. Check leftmost 8emLen-emBits bits of maskedDB are 0. */
> + /* Can only find emBits by counting the zeros on the Left. */
> +
> + /* 7. Let dbMask = MGF(H, emLen - hLen - 1). */
> + err = MGF1(ctx, H, hLen, dbMask, DB_len);
> + if (err < 0)
> + return err;
> +
> + /* 8. Let DB = maskedDB XOR dbMask. */
> + for (i = 0; i < DB_len; i++)
> + DB[i] = maskedDB[i] ^ dbMask[i];
> +
> + /* 9. Set leftmost bits in DB to zero. */
> + int z = 8 * emLen - emBits;
> + if (z > 0) {
> + if (z >= 8) {
> + DB[0] = 0;
> + } else {
> + z = 8 - z;
> + DB[0] &= (1 << z) - 1;
> + }
> + }
> +
> + /* 10. Check the left part of DB is {0,0,...,1}. */
> + for (i = 0; i < emLen - hLen - sLen - 2; i++)
> + if (DB[i] != 0)
> + return -EKEYREJECTED;
> + if (DB[i] != 0x01)
> + return -EKEYREJECTED;
> +
> + /* 11. Let salt be the last sLen octets of DB. */
> + salt = DB + DB_len - sLen;
> +
> + /* 12. Let M' be 00 00 00 00 00 00 00 00 || mHash || salt. */
> + memset(Mprime, 0, 8);
> + memcpy(Mprime + 8, mHash, hLen);
> + memcpy(Mprime + 8 + hLen, salt, sLen);
> +
> + /* 13. Let H' = Hash(M'). */
> + err = crypto_shash_digest(Hash, Mprime, 8 + hLen + sLen, Hprime);
> + if (err < 0)
> + return err;
> +
> + /* 14. Check H = H'. */
> + if (memcmp(H, Hprime, hLen) != 0)
> + return -EKEYREJECTED;
> + return 0;
> +}
> +
> +/*
> + * Perform RSASSA-PSS-VERIFY((n,e),M,S) - RFC8017 sec 8.1.2.
> + */
> +static int rsassa_pss_verify(struct crypto_sig *tfm,
> + const void *src, unsigned int slen,
> + const void *digest, unsigned int dlen,
> + const char *info)
> +{
> + struct akcipher_request *rsa_req __free(kfree) = NULL;
> + struct rsassa_pss_ctx *ctx = crypto_sig_ctx(tfm);
> + struct crypto_wait cwait;
> + struct scatterlist sg;
> + unsigned int rsa_reqsize = crypto_akcipher_reqsize(ctx->rsa);
> + char *str __free(kfree) = NULL;
> + u8 *EM;
> + int err;
> +
> + if (!info)
> + return -EINVAL;
> +
> + str = kstrdup(info, GFP_KERNEL);
> + if (!str)
> + return -ENOMEM;
> +
> + err = rsassa_pss_vinfo_parse(ctx, str);
> + if (err < 0)
> + return err;
> +
> + /* RFC8017 sec 8.1.2 step 1 - length checking */
> + if (!ctx->key_size || slen != ctx->key_size)
> + return -EINVAL;
> +
> + /* RFC8017 sec 8.1.2 step 2 - RSA verification */
> + rsa_req = kmalloc(sizeof(*rsa_req) + rsa_reqsize + ctx->key_size,
> + GFP_KERNEL);
> + if (!rsa_req)
> + return -ENOMEM;
> +
> + EM = (u8 *)(rsa_req + 1) + rsa_reqsize;
> + memcpy(EM, src, slen);
> +
> + crypto_init_wait(&cwait);
> + sg_init_one(&sg, EM, slen);
> + akcipher_request_set_tfm(rsa_req, ctx->rsa);
> + akcipher_request_set_crypt(rsa_req, &sg, &sg, slen, slen);
> + akcipher_request_set_callback(rsa_req, CRYPTO_TFM_REQ_MAY_SLEEP,
> + crypto_req_done, &cwait);
> +
> + err = crypto_akcipher_encrypt(rsa_req);
> + err = crypto_wait_req(err, &cwait);
> + if (err)
> + return err;
> +
> + /* RFC 8017 sec 8.1.2 step 3 - EMSA-PSS(M, EM, modbits-1) */
> + return emsa_pss_verify(ctx, digest, dlen, EM, slen);
> +}
> +
> +static unsigned int rsassa_pss_key_size(struct crypto_sig *tfm)
> +{
> + struct rsassa_pss_ctx *ctx = crypto_sig_ctx(tfm);
> +
> + return ctx->key_size * BITS_PER_BYTE;
> +}
> +
> +static int rsassa_pss_set_pub_key(struct crypto_sig *tfm,
> + const void *key, unsigned int keylen)
> +{
> + struct rsassa_pss_ctx *ctx = crypto_sig_ctx(tfm);
> +
> + return rsa_set_key(ctx->rsa, &ctx->key_size, RSA_PUB, key, keylen);
> +}
> +
> +static int rsassa_pss_init_tfm(struct crypto_sig *tfm)
> +{
> + struct crypto_akcipher *rsa;
> + struct rsassa_pss_ctx *ctx = crypto_sig_ctx(tfm);
> +
> + rsa = crypto_alloc_akcipher("rsa", 0, 0);
> + if (IS_ERR(rsa))
> + return PTR_ERR(rsa);
> +
> + ctx->rsa = rsa;
> + return 0;
> +}
> +
> +static void rsassa_pss_exit_tfm(struct crypto_sig *tfm)
> +{
> + struct rsassa_pss_ctx *ctx = crypto_sig_ctx(tfm);
> +
> + crypto_free_akcipher(ctx->rsa);
> +}
> +
> +struct sig_alg rsassa_pss_alg = {
> + .verify = rsassa_pss_verify,
> + .set_pub_key = rsassa_pss_set_pub_key,
> + .key_size = rsassa_pss_key_size,
> + .init = rsassa_pss_init_tfm,
> + .exit = rsassa_pss_exit_tfm,
> + .base = {
> + .cra_name = "rsassa-pss",
> + .cra_driver_name = "rsassa-pss-generic",
> + .cra_priority = 100,
> + .cra_module = THIS_MODULE,
> + .cra_ctxsize = sizeof(struct rsassa_pss_ctx),
> + },
> +};
> +
> +MODULE_ALIAS_CRYPTO("rsassa-pss");
> diff --git a/include/crypto/internal/rsa.h b/include/crypto/internal/rsa.h
> index 071a1951b992..d7f38a273949 100644
> --- a/include/crypto/internal/rsa.h
> +++ b/include/crypto/internal/rsa.h
> @@ -83,4 +83,6 @@ static inline int rsa_set_key(struct crypto_akcipher *child,
>
> extern struct crypto_template rsa_pkcs1pad_tmpl;
> extern struct crypto_template rsassa_pkcs1_tmpl;
> +extern struct sig_alg rsassa_pss_alg;
> +
> #endif
>
BR, Jarkko
