EVP_ENCRYPTINIT(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

okay so bestie, welcome to EVP_ENCRYPTINIT(3) from the Library Functions Manual — this is the libcrypto page for the EVP cipher routines, and it's lowkey a whole friend group. 🫶

💖 NAME 💖

The whole cast list on the door is giving “symmetric cipher besties assemble,” and every single one of these names is on the page:

EVP_CIPHER_CTX_new, EVP_CIPHER_CTX_reset, EVP_CIPHER_CTX_free,
EVP_CIPHER_CTX_copy, EVP_EncryptInit_ex, EVP_EncryptUpdate,
EVP_EncryptFinal_ex, EVP_DecryptInit_ex, EVP_DecryptUpdate,
EVP_DecryptFinal_ex, EVP_CipherInit_ex, EVP_CipherUpdate,
EVP_CipherFinal_ex, EVP_EncryptInit, EVP_EncryptFinal, EVP_DecryptInit,
EVP_DecryptFinal, EVP_CipherInit, EVP_CipherFinal,
EVP_CIPHER_CTX_encrypting, EVP_get_cipherbyname, EVP_get_cipherbynid,
EVP_get_cipherbyobj, EVP_CIPHER_CTX_cipher, EVP_enc_null, EVP_idea_cbc,
EVP_idea_ecb, EVP_idea_cfb64, EVP_idea_cfb, EVP_idea_ofb, EVP_bf_cbc,
EVP_bf_ecb, EVP_bf_cfb64, EVP_bf_cfb, EVP_bf_ofb, EVP_cast5_cbc,
EVP_cast5_ecb, EVP_cast5_cfb64, EVP_cast5_cfb, EVP_cast5_ofb

These are the EVP cipher routines. Periodt. 💅

✨ SYNOPSIS ✨

okay so first thing, it's the libcrypto library and you bring in the header like this:

library “libcrypto” #include <openssl/evp.h>

Then the whole function-signature moment:

EVP_CIPHER_CTX *
EVP_CIPHER_CTX_new(void);

int
EVP_CIPHER_CTX_reset(EVP_CIPHER_CTX *ctx);

void
EVP_CIPHER_CTX_free(EVP_CIPHER_CTX *ctx);

int
EVP_CIPHER_CTX_copy(EVP_CIPHER_CTX *out, const EVP_CIPHER_CTX *in);

int
EVP_EncryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
    ENGINE *engine, const unsigned char *key, const unsigned char *iv);

int
EVP_EncryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *out_len,
    const unsigned char *in, int in_len);

int
EVP_EncryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out,
    int *out_len);

int
EVP_DecryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
    ENGINE *engine, const unsigned char *key, const unsigned char *iv);

int
EVP_DecryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *out_len,
    const unsigned char *in, int in_len);

int
EVP_DecryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out,
    int *out_len);

int
EVP_CipherInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
    ENGINE *engine, const unsigned char *key, const unsigned char *iv,
    int enc);

int
EVP_CipherUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out, int *out_len,
    const unsigned char *in, int in_len);

int
EVP_CipherFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out,
    int *out_len);

int
EVP_EncryptInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
    const unsigned char *key, const unsigned char *iv);

int
EVP_EncryptFinal(EVP_CIPHER_CTX *ctx, unsigned char *out, int *out_len);

int
EVP_DecryptInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
    const unsigned char *key, const unsigned char *iv);

int
EVP_DecryptFinal(EVP_CIPHER_CTX *ctx, unsigned char *out, int *out_len);

int
EVP_CipherInit(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
    const unsigned char *key, const unsigned char *iv, int enc);

int
EVP_CipherFinal(EVP_CIPHER_CTX *ctx, unsigned char *out, int *out_len);

int
EVP_CIPHER_CTX_encrypting(const EVP_CIPHER_CTX *ctx);

const EVP_CIPHER *
EVP_get_cipherbyname(const char *name);

const EVP_CIPHER *
EVP_get_cipherbynid(int nid);

const EVP_CIPHER *
EVP_get_cipherbyobj(const ASN1_OBJECT *a);

const EVP_CIPHER *
EVP_CIPHER_CTX_cipher(const EVP_CIPHER_CTX *ctx);

🦋 DESCRIPTION 🦋

The EVP cipher routines are a high level interface to certain symmetric ciphers. That's the vibe: high-level interface to certain symmetric ciphers, and you don't gotta hand-roll the low-level stuff. 💅

🎀 context life cycle

EVP_CIPHER_CTX_new() creates a new, empty cipher context. Fresh out the box, empty, ready. ✨

EVP_CIPHER_CTX_reset() clears all information from ctx and frees all allocated memory associated with it — except the ctx object itself — such that it can be reused for another series of calls to EVP_CipherInit(), EVP_CipherUpdate(), and EVP_CipherFinal(). Reuse era, we love to see it. 🫶

EVP_CIPHER_CTX_free() clears all information from ctx and frees all allocated memory associated with it, including ctx itself. This function should be called after all operations using a cipher are complete, so sensitive information does not remain in memory. If ctx is a NULL pointer, no action occurs. No drama, no leaks. 🙈

EVP_CIPHER_CTX_copy() calls EVP_CIPHER_CTX_reset() on out and copies all the data from in to out, except that the EVP_CIPHER object used by in and any application specific data set with EVP_CIPHER_CTX_set_app_data(3) are not copied and out will point to the same two objects.

The algorithm- and implementation-specific cipher data described in EVP_CIPHER_CTX_get_cipher_data(3) is copied with malloc(3) and memcpy(3), i.e. assuming that it does not contain pointers to any sub-objects.

If the bit EVP_CIPH_CUSTOM_COPY has been set with EVP_CIPHER_meth_set_flags(3), EVP_CIPHER_CTX_ctrl(3) is called at the end with arguments in, EVP_CTRL_COPY, 0, and out such that the cipher implementation can perform further algorithm- and implementation-specific initializations after the algorithm- and implementation-specific cipher data has been copied. Among the cipher algorithms built into the library, EVP_CIPH_CUSTOM_COPY and EVP_CTRL_COPY are used by some of the ciphers documented in the EVP_aes_256_gcm(3) manual page. Very custom, very copy. 🎀

🔥 encryption squad

EVP_EncryptInit() and EVP_EncryptInit_ex() set up the cipher context ctx for encryption with cipher type. type is normally supplied by a function such as EVP_aes_256_cbc(3). key is the symmetric key to use and iv is the IV to use (if necessary). The actual number of bytes used for the key and IV depends on the cipher.

The ENGINE *engine argument is always ignored and passing NULL is recommended. It is possible to set all parameters to NULL except type in an initial call and supply the remaining parameters in subsequent calls, all of which have type set to NULL. This is done when the default cipher parameters are not appropriate. So yeah, you can split the setup across calls — iconic flexible behavior. 🌈

EVP_EncryptUpdate() encrypts in_len bytes from the buffer in and writes the encrypted version to out. This function can be called multiple times to encrypt successive blocks of data. The amount of data written depends on the block alignment of the encrypted data: as a result the amount of data written may be anything from zero bytes to (in_len + cipher_block_size - 1) so out should contain sufficient room. The actual number of bytes written is placed in *out_len. Give out room, bestie. 💖

If padding is enabled (the default) then EVP_EncryptFinal() and EVP_EncryptFinal_ex(), which behave identically, encrypt the "final" data, that is any data that remains in a partial block. It uses NOTES (aka PKCS padding). The encrypted final data is written to out which should have sufficient space for one cipher block. The number of bytes written is placed in *out_len. After this function is called, the encryption operation is finished and no further calls to EVP_EncryptUpdate() should be made.

If padding is disabled then EVP_EncryptFinal() and EVP_EncryptFinal_ex() do not encrypt any more data and return an error if any data remains in a partial block: that is if the total data length is not a multiple of the block size.

💜 decryption squad

EVP_DecryptInit(), EVP_DecryptInit_ex(), EVP_DecryptUpdate(), EVP_DecryptFinal(), and EVP_DecryptFinal_ex() are the corresponding decryption operations.

EVP_DecryptFinal() and EVP_DecryptFinal_ex() return an error code if padding is enabled and the final block is not correctly formatted.

The parameters and restrictions are identical to the encryption operations except that if padding is enabled the decrypted data buffer out passed to EVP_DecryptUpdate() should have sufficient room for (in_len + cipher_block_size) bytes unless the cipher block size is 1 in which case in_len bytes is sufficient. Tiny block size exception, noted. 🫡

🌈 generic cipher energy

EVP_CipherInit(), EVP_CipherInit_ex(), EVP_CipherUpdate(), EVP_CipherFinal(), and EVP_CipherFinal_ex() are functions that can be used for decryption or encryption. The operation performed depends on the value of the enc parameter. It should be set to 1 for encryption, 0 for decryption and -1 to leave the value unchanged (the actual value of enc being supplied in a previous call). Main character parameter behavior, honestly. ⭐

⭐ lookups & identity

EVP_get_cipherbyname(), EVP_get_cipherbynid(), and EVP_get_cipherbyobj() return an EVP_CIPHER structure when passed a cipher name, a NID or an ASN1_OBJECT structure.

EVP_CIPHER_CTX_cipher() returns the EVP_CIPHER structure when passed an EVP_CIPHER_CTX structure.

Where possible the EVP interface to symmetric ciphers should be used in preference to the low level interfaces. This is because the code then becomes transparent to the cipher used and much more flexible. Slay for portability and flexibility. 💅

🍓 PKCS padding tea

PKCS padding works by adding n padding bytes of value n to make the total length of the encrypted data a multiple of the block size. Padding is always added so if the data is already a multiple of the block size n will equal the block size. For example if the block size is 8 and 11 bytes are to be encrypted then 5 padding bytes of value 5 will be added.

When decrypting, the final block is checked to see if it has the correct form.

Although the decryption operation can produce an error if padding is enabled, it is not a strong test that the input data or key is correct. A random block has better than 1 in 256 chance of being of the correct format and problems with the input data earlier on will not produce a final decrypt error. So like, don't treat a clean decrypt as proof that everything is correct — the page literally says it's not a strong test. 💀

If padding is disabled then the decryption operation will always succeed if the total amount of data decrypted is a multiple of the block size.

EVP_get_cipherbynid() and EVP_get_cipherbyobj() are implemented as macros. iykyk. 🔥

🌸 RETURN VALUES 🌸

  • EVP_CIPHER_CTX_new() returns a pointer to a newly created EVP_CIPHER_CTX for success or NULL for failure.
  • EVP_CIPHER_CTX_reset(), EVP_CIPHER_CTX_copy(), EVP_EncryptInit_ex(), EVP_EncryptUpdate(), EVP_EncryptFinal_ex(), EVP_DecryptInit_ex(), EVP_DecryptUpdate(), EVP_DecryptFinal_ex(), EVP_CipherInit_ex(), EVP_CipherUpdate(), EVP_CipherFinal_ex(), EVP_EncryptInit(), EVP_EncryptFinal(), EVP_DecryptInit(), EVP_DecryptFinal(), EVP_CipherInit(), and EVP_CipherFinal() return 1 for success or 0 for failure.
  • EVP_CIPHER_CTX_encrypting() returns 1 if ctx is initialized for encryption or 0 otherwise, in which case it may be uninitialized or initialized for decryption.
  • EVP_get_cipherbyname(), EVP_get_cipherbynid(), and EVP_get_cipherbyobj() return an EVP_CIPHER structure or NULL on error.
  • EVP_CIPHER_CTX_cipher() returns an EVP_CIPHER structure.

🔐 CIPHER LISTING 🔐

Okay bestie, here's the cast of characters — every cipher this page knows about, with the deets ✨

  • EVP_enc_null() — the null cipher. It literally does nothing. Zero notes. It's giving "I'm just here for the vibes" 💀
  • EVP_idea_cbc(), EVP_idea_ecb(), EVP_idea_cfb64(), EVP_idea_ofb() — the IDEA encryption algorithm in CBC, ECB, CFB and OFB modes respectively. IDEA is a block cipher that operates on 64 bit blocks using a 128 bit key. Also, EVP_idea_cfb() is an alias for EVP_idea_cfb64(), implemented as a macro. Sneaky little alias, we love a shortcut 🎀
  • EVP_bf_cbc(), EVP_bf_ecb(), EVP_bf_cfb64(), EVP_bf_ofb() — Blowfish encryption algorithm in CBC, ECB, CFB and OFB modes respectively. Blowfish is a block cipher operating on 64 bit blocks using a variable key length. The default key length is 128 bits. And yes, EVP_bf_cfb() is an alias for EVP_bf_cfb64(), implemented as a macro. Fishy behavior but make it iconic 🐟
  • EVP_cast5_cbc(), EVP_cast5_ecb(), EVP_cast5_cfb64(), EVP_cast5_ofb() — CAST-128 encryption algorithm in CBC, ECB, CFB and OFB modes respectively. CAST-128 is a block cipher operating on 64 bit blocks using a variable key length. The default and maximum key length is 128 bits. EVP_cast5_cfb() is an alias for EVP_cast5_cfb64(), implemented as a macro. Very "the limit does not exist — oh wait, it does" energy 🌸

📚 the ones with their own man pages 📚

Some algorithms are so main character they got documented in separate manual pages. Here's the full roster with their block sizes and key sizes (in bits) — no gatekeeping, all the numbers are yours 💅

🌈 EXAMPLES 🌈

example one: encrypt a string with blowfish 🐡

Here's a whole function, ready to copy into your little project:

int
do_crypt(char *out_filename)
{
        unsigned char out_buf[1024];
        int out_len, tmp_len;
        /*
         * Bogus key and IV: we'd normally set these from
         * another source.
         */
        unsigned char key[] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
        unsigned char iv[] = {1,2,3,4,5,6,7,8};
        const char in_text[] = "Some Crypto Text";
        EVP_CIPHER_CTX *ctx;
        FILE *out_fileptr;

        ctx = EVP_CIPHER_CTX_new();
        EVP_EncryptInit_ex(ctx, EVP_bf_cbc(), NULL, key, iv);

        if (!EVP_EncryptUpdate(ctx, out_buf, &out_len, in_text,
            strlen(in_text))) {
                /* Error */
                EVP_CIPHER_CTX_free(ctx);
                return 0;
        }
        /*
         * Buffer passed to EVP_EncryptFinal() must be after data just
         * encrypted to avoid overwriting it.
         */
        if (!EVP_EncryptFinal_ex(ctx, out_buf + out_len, &tmp_len)) {
                /* Error */
                EVP_CIPHER_CTX_free(ctx);
                return 0;
        }
        out_len += tmp_len;
        EVP_CIPHER_CTX_free(ctx);
        /*
         * Need binary mode for fopen because encrypted data is
         * binary data. Also cannot use strlen() on it because
         * it won't be NUL terminated and may contain embedded
         * NULs.
         */
        out_fileptr = fopen(out_filename, "wb");
        if (out_fileptr == NULL) {
                /* Error */
                return 0;
        }
        fwrite(out_buf, 1, out_len, out_fileptr);
        fclose(out_fileptr);
        return 1;
}
Bestie, notice that key and IV are literally labeled "bogus" in the comments — the man page is telling you not to use those for real. Set them from another source, fr fr 🙈

And the ciphertext that comes out of the above can be decrypted with the openssl(1) utility using this command line:

openssl bf -in cipher.bin -K 000102030405060708090A0B0C0D0E0F \
           -iv 0102030405060708 -d

Like, that's it. Round trip complete, slay 💖

example two: general encrypt/decrypt with FILE I/O and AES128 🔥

This one does both directions with a 128-bit key:

int
do_crypt(FILE *in_fileptr, FILE *out_fileptr, int do_encrypt)
{
        /* Allow enough space in output buffer for additional block */
        unsigned char in_buf[1024], out_buf[1024 + EVP_MAX_BLOCK_LENGTH];
        int in_len, out_len;
        EVP_CIPHER_CTX *ctx;

        /*
         * Bogus key and IV: we'd normally set these from
         * another source.
         */
        unsigned char key[] = "0123456789abcdeF";
        unsigned char iv[] = "1234567887654321";

        ctx = EVP_CIPHER_CTX_new();
        EVP_CipherInit_ex(ctx, EVP_aes_128_cbc(), NULL, NULL, NULL,
            do_encrypt);
        EVP_CipherInit_ex(ctx, NULL, NULL, key, iv, do_encrypt);

        for (;;) {
                in_len = fread(in_buf, 1, 1024, in_fileptr);
                if (in_len <= 0)
                        break;
                if (!EVP_CipherUpdate(ctx, out_buf, &out_len, in_buf,
                    in_len)) {
                        /* Error */
                        EVP_CIPHER_CTX_free(ctx);
                        return 0;
                }
                fwrite(out_buf, 1, out_len, out_fileptr);
        }
        if (!EVP_CipherFinal_ex(ctx, out_buf, &out_len)) {
                /* Error */
                EVP_CIPHER_CTX_free(ctx);
                return 0;
        }
        fwrite(out_buf, 1, out_len, out_fileptr);

        EVP_CIPHER_CTX_free(ctx);
        return 1;
}

Notice the two-step EVP_CipherInit_ex() situation — first call sets up the cipher, second one drops in the key and IV. And EVP_MAX_BLOCK_LENGTH is doing the heavy lifting in that output buffer size, don't skip it ✨

👀 SEE ALSO 👀

All the friends of this page, in exactly the order the man page lists them — go read these next, iykyk:

BIO_f_cipher(3), evp(3), EVP_AEAD_CTX_init(3), EVP_aes_128_cbc(3), EVP_aes_128_ccm(3), EVP_aes_128_gcm(3), EVP_camellia_128_cbc(3), EVP_chacha20(3), EVP_CIPHER_CTX_ctrl(3), EVP_CIPHER_CTX_get_cipher_data(3), EVP_CIPHER_CTX_init(3), EVP_CIPHER_CTX_set_flags(3), EVP_CIPHER_nid(3), EVP_des_cbc(3), EVP_OpenInit(3), EVP_rc2_cbc(3), EVP_rc4(3), EVP_SealInit(3), EVP_sm4_cbc(3)

🕰️ HISTORY 🕰️

Okay so this is the lore drop, buckle up 🦋

EVP_EncryptInit(), EVP_EncryptUpdate(), EVP_EncryptFinal(), EVP_DecryptInit(), EVP_DecryptUpdate(), EVP_DecryptFinal(), EVP_CipherInit(), EVP_CipherUpdate(), EVP_CipherFinal(), EVP_get_cipherbyname(), EVP_idea_cbc(), EVP_idea_ecb(), EVP_idea_cfb(), and EVP_idea_ofb() first appeared in SSLeay 0.5.1.

EVP_bf_cbc(), EVP_bf_ecb(), EVP_bf_cfb(), and EVP_bf_ofb() first appeared in SSLeay 0.6.6.

EVP_get_cipherbyobj(), EVP_CIPHER_CTX_cipher(), and EVP_enc_null() first appeared in SSLeay 0.8.0.

EVP_get_cipherbynid() first appeared in SSLeay 0.8.1.

All these functions have been available since OpenBSD 2.4. Ancient. Legendary. Grandmillennial energy 👑

EVP_EncryptInit_ex(), EVP_EncryptFinal_ex(), EVP_DecryptInit_ex(), EVP_DecryptFinal_ex(), EVP_CipherInit_ex(), and EVP_CipherFinal_ex() first appeared in OpenSSL 0.9.7 and have been available since OpenBSD 3.2.

EVP_bf_cfb64(), EVP_cast5_cfb64(), and EVP_idea_cfb64() first appeared in OpenSSL 0.9.7e and have been available since OpenBSD 3.8. The 64 girlies arrived late but they arrived 😌

EVP_CIPHER_CTX_new() and EVP_CIPHER_CTX_free() first appeared in OpenSSL 0.9.8b and have been available since OpenBSD 4.5.

EVP_CIPHER_CTX_copy() first appeared in OpenSSL 1.0.0 and has been available since OpenBSD 4.9.

EVP_CIPHER_CTX_reset() first appeared in OpenSSL 1.1.0 and has been available since OpenBSD 6.3.

EVP_CIPHER_CTX_encrypting() first appeared in OpenSSL 1.1.0 and has been available since OpenBSD 6.4.

💀 BUGS 💀

Not me having to tell you this but — EVP_CIPHER_CTX_copy() may already have cleared the data in out and copied some new data into it even if it fails and returns 0.

So like, if it returns 0, don't assume your out buffer is untouched. It might already be messed with. Check your data accordingly, bestie 😭

That's the whole page! You made it to the end, I'm so proud 🫶 This one's from the Debian crew, dated June 8, 2025, and it's the EVP_ENCRYPTINIT(3) man page — go forth and encrypt responsibly, mwah 💜

📦 where this page comes from