Convert NSData to String?

I believe your "P" as the dataWithBytes param

NSData *keydata = [NSData dataWithBytes:P length:len];

should be "buf"

NSData *keydata = [NSData dataWithBytes:buf length:len];

since i2d_PrivateKey puts the pointer to the output buffer p at the end of the buffer and waiting for further input, and buf is still pointing to the beginning of your buffer.

The following code works for me where pkey is a pointer to an EVP_PKEY:

unsigned char *buf, *pp;
int len = i2d_PrivateKey(pkey, NULL);
buf = OPENSSL_malloc(len); 
pp = buf;
i2d_PrivateKey(pkey, &pp);

NSData* pkeyData = [NSData dataWithBytes:(const void *)buf length:len];
DLog(@"Private key in hex (%d): %@", len, pkeyData);

You can use an online converter to convert your binary data into base 64 (http://tomeko.net/online_tools/hex_to_base64.php?lang=en) and compare it to the private key in your cert file after using the following command and checking the output of mypkey.pem:

openssl pkcs12 -in myCert.p12 -nocerts -nodes -out mypkey.pem

I referenced your question and this EVP function site for my answer.

Objective-C

You can use (see NSString Class Reference)

- (id)initWithData:(NSData *)data encoding:(NSStringEncoding)encoding

Example:

NSString *myString = [[NSString alloc] initWithData:myData encoding:NSUTF8StringEncoding];

Remark: Please notice the NSData value must be valid for the encoding specified (UTF-8 in the example above), otherwise nil will be returned:

Returns nil if the initialization fails for some reason (for example if data does not represent valid data for encoding).

Prior Swift 3.0

String(data: yourData, encoding: NSUTF8StringEncoding)

Swift 3.0 Onwards

String(data: yourData, encoding: .utf8)

See String#init(data:encoding:) Reference

Prior Swift 3.0 :

String(data: yourData, encoding: NSUTF8StringEncoding)

For Swift 4.0:

String(data: yourData, encoding: .utf8)

Swift 5:

String(data: data!, encoding: String.Encoding.utf8)