PEM_READ(3) | Library Functions Manual | PEM_READ(3)

# NAME

`PEM_write`, `PEM_write_bio`, `PEM_read`, `PEM_read_bio`, `PEM_get_EVP_CIPHER_INFO`, `PEM_do_header`, `PEM_def_callback`, `pem_password_cb` — PEM encoding routines

# SYNOPSIS

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

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_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 *userdata);

int
`PEM_def_callback`(char *password, int size, int verify, void *userdata);

typedef int
`pem_password_cb`(char *password, int size, int verify, void *userdata);

# DESCRIPTION

These functions read and write PEM-encoded objects, using the PEM type name, any additional header information, and the raw data of length len.

PEM is the binary content encoding first defined in IETF RFC 1421. The content is a series of base64-encoded lines, surrounded by begin/end markers each on their own line. For example:

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

Optional header line(s) may appear after the begin line, and their existence depends on the type of object being written or read.

`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.

The final data buffer is typically an ASN.1 object which can be decoded with the `d2i_*`() function appropriate to the type name; see d2i_X509(3) for examples.

`PEM_read`() reads from the file fp, while `PEM_read_bio`() reads from the BIO bp. Both skip any non-PEM data that precedes the start of the next PEM object. When an object is successfully retrieved, the type name from the "----BEGIN <type>-----" is returned via the name argument, any encapsulation headers are returned in header, and the base64-decoded content and its length are returned via data and len, respectively. The name, header, and data pointers should be freed by the caller when no longer needed.

The remaining functions are deprecated because the underlying PEM encryption format is obsolete and should be avoided. It uses an encryption format with an OpenSSL-specific key-derivation function, which employs MD5 with an iteration count of 1. 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).

`PEM_get_EVP_CIPHER_INFO`() can be used to determine 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 and 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 when the cipher is unknown or some internal error happens, 0 is returned.

`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 a preceding call to `PEM_get_EVP_CIPHER_INFO`(). If that structure indicates the absence of encryption, `PEM_do_header`() returns successfully without taking any action. The data and len arguments are used both to pass in the encrypted data that was returned in the same arguments from the preceding call to `PEM_read`() or `PEM_read_bio`() and to pass out the decrypted data.

The callback function cb is used to obtain the encryption password; if cb is `NULL`, `PEM_def_callback`() is used instead. The password buffer needs to be at least size bytes long. Unless userdata is `NULL`, `PEM_def_callback`() ignores the verify argument and copies the NUL-terminated byte string userdata to password without a terminating NUL byte, silently truncating the copy to at most size bytes. If userdata is `NULL`, `PEM_def_callback`() instead prompts the user for the password with echoing turned off by calling EVP_read_pw_string_min(3) internally. In this case, the size is silently reduced to at most `BUFSIZ` and at most size - 1 bytes are accepted from the user and copied into the byte string buffer password. A callback function cb supplied by the application may use userdata for a different purpose than `PEM_def_callback`() does, e.g., as auxiliary data to use while acquiring the password. For example, a GUI application might pass a window handle. If the verify flag is non-zero, the user is prompted twice for the password to make typos less likely and it is checked that both inputs agree. This flag is not set by `PEM_do_header`() nor by other read functions, but it is typically set by write functions.

If the data is a priori known to not be encrypted, then neither `PEM_get_EVP_CIPHER_INFO`() nor `PEM_do_header`() need to be called.

# RETURN VALUES

`PEM_read`() and `PEM_read_bio`() return 1 on success or 0 on failure. The latter includes the case 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) and ERR_GET_REASON(3).

`PEM_get_EVP_CIPHER_INFO`() and `PEM_do_header`() return 1 on success or 0 on failure. The data is likely meaningless if these functions fail.

`PEM_def_callback`() returns the number of bytes stored into buf or a negative value on failure, and cb is expected to behave in the same way. If userdata is `NULL`, `PEM_def_callback`() fails if num is less than 5 or if an error occurs trying to prompt the user for the password. Otherwise, it fails when num is negative. The details of the circumstances that cause cb to fail may differ.

# SEE ALSO

crypto(3), d2i_PKCS8PrivateKey_bio(3), PEM_ASN1_read(3), PEM_bytes_read_bio(3), PEM_read_bio_PrivateKey(3), PEM_read_SSL_SESSION(3), PEM_write_bio_CMS_stream(3), PEM_write_bio_PKCS7_stream(3), PEM_X509_INFO_read_bio(3)

# HISTORY

`PEM_write`(), `PEM_read`(), and `PEM_do_header`() appeared in SSLeay 0.4 or earlier. `PEM_get_EVP_CIPHER_INFO`() first appeared in SSLeay 0.5.1. `PEM_write_bio`() and `PEM_read_bio`() first appeared in SSLeay 0.6.0. These functions have been available since OpenBSD 2.4.

`PEM_def_callback`() first appeared in OpenSSL 0.9.7 and has been available since OpenBSD 3.2.

July 16, 2025 | Debian