Convert NSData of Image from database to NSString in iPhone

I am converting NSData of Image from database to NSString in Iphone

Here is my code

imageData1 = [[NSData alloc] initWithBytes:sqlite3_column_blob(compiledStatement, 3) length: sqlite3_column_bytes(compiledStatement, 3)];           
NSString * Str1 = [NSString base64StringFromData: imageData1 length: [imageData1 length]];
NSData *data1=[NSData base64DataFromString:Str1];
NSString* newStr1 = [[NSString alloc] 开发者_运维问答initWithData:data1 encoding:NSUTF8StringEncoding];

the length of data is 6511 , but the newstr1 is nil

With reference to this question: I can't create a UTF-8 encoded string out of just any arbitrary binary data from the database - the data needs to actually be UTF-8 encoded. I have aslo done base 64 encoding and decoding. How can I solve this issue. I need it in the format of NSString only, to give it as input to a csv file?

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I don't have ample knowledge of character encoding, but would it not be sufficient to just use the Base 64 encoded string (Str1) in your output?

I fear that re-encoding Str1 as data1 brings you right back to square 1 and undoes any Base 64 encoding done in the line earlier.