[PATCH v3 14/18] crypto: Implement MGF1 Mask Generation Function for RSASSA-PSS

Varad Gautam
2021-04-20T11:47:04Z

This generates a "mask" byte array of size mask_len bytes as a
concatenation of digests, where each digest is calculated on a
concatenation of an input seed and a running counter to fill up
mask_len bytes - as described by RFC8017 sec B.2.1. "MGF1".

The mask is useful for RSA signing/verification process with
encoding RSASSA-PSS.

Reference: https://tools.ietf.org/html/rfc8017#appendix-B.2.1
Signed-off-by: Varad Gautam <varad.gautam@suse.com>
---
crypto/rsa-psspad.c | 54 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)

diff --git a/crypto/rsa-psspad.c b/crypto/rsa-psspad.c
index 0a9c0f9e9f0fe..990c2cda552a7 100644
--- a/crypto/rsa-psspad.c
+++ b/crypto/rsa-psspad.c
@@ -70,6 +70,60 @@ static int psspad_set_sig_params(struct crypto_akcipher *tfm,
return 0;
}

+/* MGF1 per RFC8017 B.2.1. */
+static int pkcs1_mgf1(u8 *seed, unsigned int seed_len,
+ struct shash_desc *desc,
+ u8 *mask, unsigned int mask_len)
+{
+ unsigned int pos, h_len, i, c;
+ u8 *tmp;
+ int ret = 0;
+
+ h_len = crypto_shash_digestsize(desc->tfm);
+
+ pos = i = 0;
+ while ((i < (mask_len / h_len) + 1) && pos < mask_len) {
+ /* Compute T = T || Hash(mgfSeed || C) into mask at pos. */
+ c = cpu_to_be32(i);
+
+ ret = crypto_shash_init(desc);
+ if (ret < 0)
+ goto out_err;
+
+ ret = crypto_shash_update(desc, seed, seed_len);
+ if (ret < 0)
+ goto out_err;
+
+ ret = crypto_shash_update(desc, (u8 *) &c, sizeof(c));
+ if (ret < 0)
+ goto out_err;
+
+ if (mask_len - pos >= h_len) {
+ ret = crypto_shash_final(desc, mask + pos);
+ pos += h_len;
+ } else {
+ tmp = kzalloc(h_len, GFP_KERNEL);
+ if (!tmp) {
+ ret = -ENOMEM;
+ goto out_err;
+ }
+ ret = crypto_shash_final(desc, tmp);
+ /* copy the last hash */
+ memcpy(mask + pos, tmp, mask_len - pos);
+ kfree(tmp);
+ pos = mask_len;
+ }
+ if (ret < 0) {
+ goto out_err;
+ }
+
+ i++;
+ }
+
+out_err:
+ return ret;
+}
+
static int psspad_s_v_e_d(struct akcipher_request *req)
{
return -EOPNOTSUPP;
--
2.30.2