Improve and document low-level PEM read routines

PEM_read(), PEM_read_bio(), PEM_get_EVP_CIPHER_INFO() and
PEM_do_header().

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>

1 files changed, 90 insertions, 0 deletions

diff --git a/doc/crypto/pem_read.pod b/doc/crypto/pem_read.pod
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+=pod
+
+=head1 NAME
+
+PEM_read, PEM_read_bio, PEM_do_header - low-level PEM routines
+
+=head1 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);
+
+=head1 DESCRIPTION
+
+These functions read and decode PEM-encoded objects, returning the
+PEM type B<name>, any encapsulation B<header> and the decoded
+B<data> of length B<len>.
+
+PEM_read() reads from the stdio file handle B<fp>, while PEM_read_bio() reads
+from the BIO B<bio>.
+Both skip any non-PEM data that precedes the start of the next PEM object.
+When an object is successfuly retrieved, the type name from the "----BEGIN
+<type>-----" is returned via the B<name> argument, any encapsulation headers
+are returned in B<header> and the base64-decoded content and its length are
+returned via B<data> and B<len> respectively.
+The B<name>, B<header> and B<data> pointers are allocated via OPENSSL_malloc()
+and should be freed by the caller via OPENSSL_free() when no longer needed.
+
+PEM_get_EVP_CIPHER_INFO() can be used to determine the B<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 structure of type B<EVP_CIPHER_INFO> via the
+B<cinfo> argument and the B<header> returned via PEM_read() or PEM_read_bio().
+If the call is succesful 1 is retured and the cipher and IV are stored at the
+address pointed to by B<cinfo>.
+When the header is malformed, or not supported or when the cipher is unknown
+or some internal error happens 0 is returned.
+This function is deprecated, see B<NOTES> below.
+
+PEM_do_header() can then be used to decrypt the data if the header
+indicates encryption.
+The B<cinfo> argument is a pointer to the structure initialized by the previous
+call to PEM_get_EVP_CIPHER_INFO().
+The B<data> and B<len> arguments are those returned by the previous call to
+PEM_read() or PEM_read_bio().
+The B<cb> and B<u> arguments make it possible to override the default password
+prompt function as described in L<pem(3)>.
+On successful completion the B<data> is decrypted in place, and B<len> is
+updated to indicate the plaintext length.
+This function is deprecated, see B<NOTES> below.
+
+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.
+
+The final B<data> buffer is typically an ASN.1 object which can be decoded with
+the B<d2i> function appropriate to the type B<name>.
+
+=head1 RETURN VALUES
+
+PEM_read() and PEM_read_bio() return 1 on success and 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 B<PEM_R_NO_START_LINE>, which indicates that no more
+PEM objects were found.  See L<ERR_peek_last_error(3)>, L<ERR_GET_REASON(3)>.
+
+PEM_get_EVP_CIPHER_INFO() and PEM_do_header() return 1 on success, and 0 on
+failure.
+The B<data> is likely meaningless if these functions fail.
+
+=head1 NOTES
+
+The PEM_get_EVP_CIPHER_INFO() and PEM_do_header() functions are deprecated.
+This is 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 L<pkcs8(1)> and L<pem(3)> and L<d2i_PKCS8PrivateKey_bio(3)>.
+
+=head1 SEE ALSO
+
+L<pem(3)>, L<ERR_peek_last_error(3)>, L<ERR_GET_LIB(3)>, L<pkcs8(1)>,
+L<d2i_PKCS8PrivateKey_bio(3)>.