PEM_READ(3ossl)

💅 PEM_READ 💅

okay so bestie, welcome to the OpenSSL PEM encoding page — this is the manual for PEM_read, PEM_read_bio, PEM_do_header, PEM_get_EVP_CIPHER_INFO, PEM_write, PEM_write_bio, PEM_ASN1_write, PEM_ASN1_write_bio, and PEM_ASN1_write_bio_ctx, a.k.a. the PEM encoding routines. yes that's a lot of names, no i did not forget any, we're getting through all of them together 🫶

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✨ SYNOPSIS ✨

#include <openssl/pem.h>

int PEM_read(FILE *fp, char **name, char **header,
             unsigned char **data, long *len);
int PEM_read_bio(BIO *bp, char **name, char **header,
                 unsigned char **data, long *len);

int PEM_get_EVP_CIPHER_INFO(char *header, EVP_CIPHER_INFO *cinfo);
int PEM_do_header(EVP_CIPHER_INFO *cinfo, unsigned char *data, long *len,
                  pem_password_cb *cb, void *u);

int PEM_write(FILE *fp, const char *name, const char *header,
              const unsigned char *data, long len);
int PEM_write_bio(BIO *bp, const char *name, const char *header,
                  const unsigned char *data, long len);
int PEM_ASN1_write(i2d_of_void *i2d, const char *name, FILE *fp,
                   const void *x, const EVP_CIPHER *enc,
                   const unsigned char *kstr, int klen,
                   pem_password_cb *callback, void *u);
int PEM_ASN1_write_bio(i2d_of_void *i2d, const char *name, BIO *bp,
                       const void *x, const EVP_CIPHER *enc,
                       const unsigned char *kstr, int klen,
                       pem_password_cb *callback, void *u);
int PEM_ASN1_write_bio_ctx(OSSL_i2d_of_void_ctx *i2d, void *vctx,
                           const char *name, BIO *bp, const void *x,
                           const EVP_CIPHER *enc, const unsigned char *kstr,
                           int klen, pem_password_cb *callback, void *u);

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🌸 DESCRIPTION 🌸

So these functions read and write PEM-encoded objects, and the whole vibe is: you got a PEM type name, you got any extra header info, and you got the raw data of length len. That's the trio. 🎀

PEM is the term for a binary content encoding that was first defined in IETF RFC 1421. The content is literally just a series of base64-encoded lines, wrapped in begin/end markers, each on its own line. It's giving:

-----BEGIN PRIVATE KEY-----
MIICdg....
... bhTQ==
-----END PRIVATE KEY-----

Optional header line(s) can show up after the begin line, and whether they exist totally depends on the type of object being written or read. no cap. 🌈

PEM_write() writes to the file fp, while PEM_write_bio() writes to the BIO bp. The name is the name to use in the marker, the header is the header value or NULL, and data and len specify the data and its length. slay 💅

That final data buffer is typically an ASN.1 object, which you can decode with the d2i function appropriate to the type name — go peep d2i_X509(3) for examples, iykyk.

PEM_read() reads from the file fp, while PEM_read_bio() reads from the BIO bp. Both of them skip any non-PEM data that comes before the start of the next PEM object (love that for us, so forgiving ✨). When an object is successfully retrieved:

• the type name from the "----BEGIN <type>-----" comes back via the name argument
• any encapsulation headers come back in header
• the base64-decoded content and its length come back via data and len respectively

The name, header and data pointers are allocated via OPENSSL_malloc() and should be freed by the caller via OPENSSL_free() when no longer needed. do NOT forget to free them bestie 😭

PEM_get_EVP_CIPHER_INFO() can be used to determine whether the data returned by PEM_read() or PEM_read_bio() is encrypted, and to retrieve the associated cipher and IV. The caller passes a pointer to a structure of type EVP_CIPHER_INFO via the cinfo argument, plus the header returned via PEM_read() or PEM_read_bio(). If the call is successful, 1 is returned and the cipher and IV are stored at the address pointed to by cinfo. When the header is malformed, or not supported, or the cipher is unknown, or some internal error happens — 0 is returned. 🫠 Also: this function is deprecated, see NOTES below.

PEM_do_header() can then be used to decrypt the data if the header indicates encryption. The cinfo argument is a pointer to the structure initialized by the previous call to PEM_get_EVP_CIPHER_INFO(). The data and len arguments are those returned by the previous call to PEM_read() or PEM_read_bio(). The cb and u arguments make it possible to override the default password prompt function, as described in PEM_read_PrivateKey(3). On successful completion the data is decrypted in place, and len is updated to indicate the plaintext length. 💖 This function is also deprecated, see NOTES below.

And like — if the data is a priori known to not be encrypted, then neither PEM_do_header() nor PEM_get_EVP_CIPHER_INFO() need be called. skip the whole step, main character energy 👑

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⭐ RETURN VALUES ⭐

PEM_read() and PEM_read_bio() return 1 on success and 0 on failure — and the failure case includes when no more PEM objects remain in the input file. To distinguish end of file from more serious errors, the caller must peek at the error stack and check for PEM_R_NO_START_LINE, which indicates that no more PEM objects were found. See ERR_peek_last_error(3), ERR_GET_REASON(3).

PEM_get_EVP_CIPHER_INFO() and PEM_do_header() return 1 on success, and 0 on failure. And like… the data is likely meaningless if these functions fail 💀 so don't go using it anyway.

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🦋 NOTES 🦋

The PEM_get_EVP_CIPHER_INFO() and PEM_do_header() functions are deprecated. Why? Because the underlying PEM encryption format is obsolete and should be avoided. Fr fr — it uses an encryption format with an OpenSSL-specific key-derivation function, which employs MD5 with an iteration count of 1! 😭😭 the disrespect. Instead, private keys should be stored in PKCS#8 form, with a strong PKCS#5 v2.0 PBE. See PEM_write_PrivateKey(3) and d2i_PKCS8PrivateKey_bio(3). do it the right way bestie 🫶

PEM_do_header() makes no assumption regarding the pass phrase received from the password callback. It will simply be treated as a byte sequence. simple as that ✨

PEM_write() and PEM_write_bio() return the number of encoded bytes (not counting the PEM header and end marker) written on success, or 0 on failure.

PEM_ASN1_write_bio() and PEM_ASN1_write_bio_ctx() return 1 on success and 0 on failure. The latter function passes an additional application-provided context value to the i2d function that serialises the input ASN.1 object. 💜

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👑 SEE ALSO 👑

ERR_peek_last_error(3), ERR_GET_LIB(3), d2i_PKCS8PrivateKey_bio(3), passphrase-encoding(7)

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🌈 HISTORY 🌈

The PEM_ASN1_write_bio_ctx() function was added in OpenSSL 3.5. new drop, we love to see it 🎀

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💖 COPYRIGHT 💖

Copyright 1998-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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okay that's the whole page bestie, you ate that 🍓 this was OpenSSL 3.5.6, dated 2026-04-07 — go forth and PEM responsibly, mwah 💅✨