diff --git a/decrepit/cfb/cfb.c b/decrepit/cfb/cfb.c
index d3a1761..441ebe6 100644
--- a/decrepit/cfb/cfb.c
+++ b/decrepit/cfb/cfb.c
@@ -57,4 +57,12 @@
     NULL /* cleanup */,  NULL /* ctrl */,
 };
 
+static const EVP_CIPHER aes_256_cfb128 = {
+    NID_aes_256_cfb128,  1 /* block_size */,  32 /* key_size */,
+    16 /* iv_len */,     sizeof(EVP_CFB_CTX), EVP_CIPH_CFB_MODE,
+    NULL /* app_data */, aes_cfb_init_key,    aes_cfb128_cipher,
+    NULL /* cleanup */,  NULL /* ctrl */,
+};
+
 const EVP_CIPHER *EVP_aes_128_cfb128(void) { return &aes_128_cfb128; }
+const EVP_CIPHER *EVP_aes_256_cfb128(void) { return &aes_256_cfb128; }
diff --git a/decrepit/cfb/cfb_test.cc b/decrepit/cfb/cfb_test.cc
index 52c46ab..ce55ba9 100644
--- a/decrepit/cfb/cfb_test.cc
+++ b/decrepit/cfb/cfb_test.cc
@@ -19,9 +19,9 @@
 #include "../../crypto/internal.h"
 #include "../../crypto/test/test_util.h"
 
-
 struct CFBTestCase {
-  uint8_t key[16];
+  size_t key_len;
+  uint8_t key[32];
   uint8_t iv[16];
   uint8_t plaintext[16*4];
   uint8_t ciphertext[16*4];
@@ -31,7 +31,8 @@
   {
     // This is the test case from
     // http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf,
-    // section F.3.13.
+    // section F.3.13, for CFB128-AES128
+    16,
     {0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6, 0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c},
     {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f},
     {0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
@@ -43,6 +44,23 @@
      0x26, 0x75, 0x1f, 0x67, 0xa3, 0xcb, 0xb1, 0x40, 0xb1, 0x80, 0x8c, 0xf1, 0x87, 0xa4, 0xf4, 0xdf,
      0xc0, 0x4b, 0x05, 0x35, 0x7c, 0x5d, 0x1c, 0x0e, 0xea, 0xc4, 0xc6, 0x6f, 0x9f, 0xf7, 0xf2, 0xe6},
   },
+  {
+    // This is the test case from
+    // http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf,
+    // section F.3.17, CFB128-AES256
+    32,
+    {0x60, 0x3d, 0xeb, 0x10, 0x15, 0xca, 0x71, 0xbe, 0x2b, 0x73, 0xae, 0xf0, 0x85, 0x7d, 0x77, 0x81,
+     0x1f, 0x35, 0x2c, 0x07, 0x3b, 0x61, 0x08, 0xd7, 0x2d, 0x98, 0x10, 0xa3, 0x09, 0x14, 0xdf, 0xf4},
+    {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f},
+    {0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
+     0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, 0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
+     0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11, 0xe5, 0xfb, 0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef,
+     0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17, 0xad, 0x2b, 0x41, 0x7b, 0xe6, 0x6c, 0x37, 0x10},
+    {0xdc, 0x7e, 0x84, 0xbf, 0xda, 0x79, 0x16, 0x4b, 0x7e, 0xcd, 0x84, 0x86, 0x98, 0x5d, 0x38, 0x60,
+     0x39, 0xff, 0xed, 0x14, 0x3b, 0x28, 0xb1, 0xc8, 0x32, 0x11, 0x3c, 0x63, 0x31, 0xe5, 0x40, 0x7b,
+     0xdf, 0x10, 0x13, 0x24, 0x15, 0xe5, 0x4b, 0x92, 0xa1, 0x3e, 0xd0, 0xa8, 0x26, 0x7a, 0xe2, 0xf9,
+     0x75, 0xa3, 0x85, 0x74, 0x1a, 0xb9, 0xce, 0xf8, 0x20, 0x31, 0x62, 0x3d, 0x55, 0xb1, 0xe4, 0x71},
+  },
 };
 
 TEST(CFBTest, TestVectors) {
@@ -56,8 +74,14 @@
 
     for (size_t stride = 1; stride <= input_len; stride++) {
       bssl::ScopedEVP_CIPHER_CTX ctx;
-      ASSERT_TRUE(EVP_EncryptInit_ex(ctx.get(), EVP_aes_128_cfb128(), nullptr,
-                                     test.key, test.iv));
+      if (test.key_len == 16) {
+        ASSERT_TRUE(EVP_EncryptInit_ex(ctx.get(), EVP_aes_128_cfb128(), nullptr,
+                                      test.key, test.iv));
+      } else {
+        assert(test.key_len == 32);
+        ASSERT_TRUE(EVP_EncryptInit_ex(ctx.get(), EVP_aes_256_cfb128(), nullptr,
+                                      test.key, test.iv));
+      }
 
       size_t done = 0;
       while (done < input_len) {
@@ -78,8 +102,14 @@
     }
 
     bssl::ScopedEVP_CIPHER_CTX decrypt_ctx;
-    ASSERT_TRUE(EVP_DecryptInit_ex(decrypt_ctx.get(), EVP_aes_128_cfb128(),
-                                   nullptr, test.key, test.iv));
+    if (test.key_len == 16) {
+      ASSERT_TRUE(EVP_DecryptInit_ex(decrypt_ctx.get(), EVP_aes_128_cfb128(),
+                                    nullptr, test.key, test.iv));
+    } else {
+      assert(test.key_len == 32);
+      ASSERT_TRUE(EVP_DecryptInit_ex(decrypt_ctx.get(), EVP_aes_256_cfb128(),
+                                    nullptr, test.key, test.iv));
+    }
 
     std::unique_ptr<uint8_t[]> plaintext(new uint8_t[input_len]);
     int num_bytes;
diff --git a/include/openssl/cipher.h b/include/openssl/cipher.h
index 17b7b91..d22a6c2 100644
--- a/include/openssl/cipher.h
+++ b/include/openssl/cipher.h
@@ -425,6 +425,9 @@
 // EVP_aes_128_cfb128 is only available in decrepit.
 OPENSSL_EXPORT const EVP_CIPHER *EVP_aes_128_cfb128(void);
 
+// EVP_aes_256_cfb128 is only available in decrepit.
+OPENSSL_EXPORT const EVP_CIPHER *EVP_aes_256_cfb128(void);
+
 // EVP_bf_ecb is Blowfish in ECB mode and is only available in decrepit.
 OPENSSL_EXPORT const EVP_CIPHER *EVP_bf_ecb(void);