Clean up EVP_DecryptFinal_ex()

Rework the code to use the usual variable names, return early if we
have block size 1 and unindent the remainder of the code for block
sizes 8 and 16. Rework the padding check to be less acrobatic and
copy the remainder of the plain text into out using memcpy() rather
than a for loop.

input/ok jsing

1 files changed, 35 insertions, 32 deletions

diff --git a/src/lib/libcrypto/evp/evp_enc.c b/src/lib/libcrypto/evp/evp_enc.c
index b00150513c..2bcef15fc2 100644
--- a/src/lib/libcrypto/evp/evp_enc.c
+++ b/src/lib/libcrypto/evp/evp_enc.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: evp_enc.c,v 1.65 2023/12/20 10:42:43 tb Exp $ */
+/* $OpenBSD: evp_enc.c,v 1.66 2023/12/20 11:01:34 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -487,50 +487,53 @@ EVP_DecryptFinal(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl)
 int
 EVP_DecryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl)
 {
-        int i, n;
-        unsigned int b;
+        const int block_size = ctx->cipher->block_size;
+        int buf_offset = ctx->buf_len;
+        int i, pad, plain_len;
 
         *outl = 0;
 
         if ((ctx->cipher->flags & EVP_CIPH_FLAG_CUSTOM_CIPHER) != 0)
                 return evp_cipher(ctx, out, outl, NULL, 0);
 
-        b = ctx->cipher->block_size;
-        if (ctx->flags & EVP_CIPH_NO_PADDING) {
-                if (ctx->buf_len) {
+        if ((ctx->flags & EVP_CIPH_NO_PADDING) != 0) {
+                if (buf_offset != 0) {
                         EVPerror(EVP_R_DATA_NOT_MULTIPLE_OF_BLOCK_LENGTH);
                         return 0;
                 }
-                *outl = 0;
                 return 1;
         }
-        if (b > 1) {
-                if (ctx->buf_len || !ctx->final_used) {
-                        EVPerror(EVP_R_WRONG_FINAL_BLOCK_LENGTH);
-                        return (0);
-                }
-                if (b > sizeof ctx->final) {
-                        EVPerror(EVP_R_BAD_BLOCK_LENGTH);
-                        return 0;
-                }
-                n = ctx->final[b - 1];
-                if (n == 0 || n > (int)b) {
+
+        if (block_size == 1)
+                return 1;
+
+        if (buf_offset != 0 || !ctx->final_used) {
+                EVPerror(EVP_R_WRONG_FINAL_BLOCK_LENGTH);
+                return 0;
+        }
+
+        if (block_size > sizeof(ctx->final)) {
+                EVPerror(EVP_R_BAD_BLOCK_LENGTH);
+                return 0;
+        }
+
+        pad = ctx->final[block_size - 1];
+        if (pad <= 0 || pad > block_size) {
+                EVPerror(EVP_R_BAD_DECRYPT);
+                return 0;
+        }
+        plain_len = block_size - pad;
+        for (i = plain_len; i < block_size; i++) {
+                if (ctx->final[i] != pad) {
                         EVPerror(EVP_R_BAD_DECRYPT);
-                        return (0);
-                }
-                for (i = 0; i < n; i++) {
-                        if (ctx->final[--b] != n) {
-                                EVPerror(EVP_R_BAD_DECRYPT);
-                                return (0);
-                        }
+                        return 0;
                 }
-                n = ctx->cipher->block_size - n;
-                for (i = 0; i < n; i++)
-                        out[i] = ctx->final[i];
-                *outl = n;
-        } else
-                *outl = 0;
-        return (1);
+        }
+
+        memcpy(out, ctx->final, plain_len);
+        *outl = plain_len;
+
+        return 1;
 }
 
 EVP_CIPHER_CTX *