How to encrypt a large file in openssl using public key

Question

How can I encrypt a large file with a public key so that no one other than who has the private key be able to decrypt it?

I can make RSA public and private keys but when it comes to encrypting a large file using this command:

openssl rsautl -encrypt -pubin -inkey public.pem -in myLargeFile.xml -out myLargeFile_encrypted.xml

and how can i perform the decryption also....

i create my private and public key by the following commands

openssl genrsa -out private.pem 1024
openssl rsa -in private.pem -out public.pem -outform PEM -pubout

I get this error:

RSA operation error
3020:error:0406D06E:rsa routines:RSA_padding_add_PKCS1_type_2:data too large for key size:.\crypto\rsa\rsa_pk1.c:151:

I tried to make keys with sizes from 1024 to 1200 bits, no luck, same error

score 90 · Accepted Answer · edited Mar 04 '14 at 21:47

90

Public-key crypto is not for encrypting arbitrarily long files. One uses a symmetric cipher (say AES) to do the normal encryption. Each time a new random symmetric key is generated, used, and then encrypted with the RSA cipher (public key). The ciphertext together with the encrypted symmetric key is transferred to the recipient. The recipient decrypts the symmetric key using his private key, and then uses the symmetric key to decrypt the message.

The private key is never shared, only the public key is used to encrypt the random symmetric cipher.

edited Mar 04 '14 at 21:47

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answered Aug 22 '11 at 10:59

n. m. could be an AI

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But the point of asymmetric cryptography is prevent sharing of private secret,this approach leads to sharing of the key even though its encrypted with asymmetric cryptography.Is there any advantage in using asymmetric cryptography like RSA to encrypt the shared secret vs symmetric algorithms – techno Oct 24 '13 at 12:21
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@techno: you are not sharing any secrets. You generate a one-time random key, use it to encrypt a single message, and throw it away. The key you are sending with the message is valid only for that message. – n. m. could be an AI Oct 24 '13 at 12:43
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Yeah,but the one time key for the particular message is shared right?Is there any advantage in using asymmetric cryptography like RSA to encrypt the shared secret vs symmetric algorithms – techno Oct 24 '13 at 13:06
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Yes. You also share the message itself. If you are afraid of sharing the key, don't share the message either. – n. m. could be an AI Oct 24 '13 at 13:12
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@n.m. "Public-key crypto is not for encrypting arbitrarily long files.", strictly from a performance point of view? Using the public key of the *recipient* (or whoever is supposed to read the file) to RSA-encrypt the whole file would bring other disadvantages? – cYrus Oct 04 '15 at 11:01
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@cYrus yes, performance would be very poor. I don't know of other disadvantages. – n. m. could be an AI Oct 04 '15 at 12:06
One thing that confuses me is: why use a public key? Are you not just shifting the responsibility of the private key to the public key now? If you encrypt with the public key what security would not using the private key be added? I cannot really see any personally since it seems like the public key now does the job of the private key, so if you had that you could decrypt the password and the file. As such the private key becomes pointless in everything but generating a public key, right? – Sammaye Aug 23 '17 at 14:54
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@Sammaye Discovering public key cryptography can be lots of fun, but why select this humble six year old answer as your starting point? Here's [the wikipedia article](https://en.wikipedia.org/wiki/Public-key_cryptography). If you have questions, there's an [entire SE site](https://crypto.stackexchange.com/) dedicated to crypto. – n. m. could be an AI Aug 23 '17 at 17:46
I'm mostly here since Google loves this question. But I actually understand now, even though you encrypt with the public key you can only decrypt with the private key, so the public key contains a cipher to use for encryption but it cannot contain the means by which to understand how to reverse engineer that cipher – Sammaye Aug 23 '17 at 18:59

Jakub Bouček · Answer 2 · 2014-10-10T08:58:49.920

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Solution for safe and high secured encode anyone file in OpenSSL and command-line:

You should have ready some X.509 certificate for encrypt files in PEM format.

Encrypt file:

openssl smime -encrypt -binary -aes-256-cbc -in plainfile.zip -out encrypted.zip.enc -outform DER yourSslCertificate.pem

What is what:

smime - ssl command for S/MIME utility (smime(1))
-encrypt - chosen method for file process
-binary - use safe file process. Normally the input message is converted to "canonical" format as required by the S/MIME specification, this switch disable it. It is necessary for all binary files (like a images, sounds, ZIP archives).
-aes-256-cbc - chosen cipher AES in 256 bit for encryption (strong). If not specified 40 bit RC2 is used (very weak). (Supported ciphers)
-in plainfile.zip - input file name
-out encrypted.zip.enc - output file name
-outform DER - encode output file as binary. If is not specified, file is encoded by base64 and file size will be increased by 30%.
yourSslCertificate.pem - file name of your certificate's. That should be in PEM format.

That command can very effectively a strongly encrypt big files regardless of its format.
Known issue: Something wrong happens when you try encrypt huge file (>600MB). No error thrown, but encrypted file will be corrupted. Always verify each file! (or use PGP - that has bigger support for files encryption with public key)

Decrypt file:

openssl smime -decrypt -binary -in encrypted.zip.enc -inform DER -out decrypted.zip -inkey private.key -passin pass:your_password

What is what:

-inform DER - same as -outform above
-inkey private.key - file name of your private key. That should be in PEM format and can be encrypted by password.
-passin pass:your_password - your password for private key encrypt. (passphrase arguments)

edited Oct 10 '14 at 08:58

answered Sep 02 '12 at 05:38

Jakub Bouček

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When I do the command above I get a `Usage smime [options] yourSslCertificate.pem ...` with all the smime options. – Brian Armstrong Jun 25 '14 at 17:34
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Turned out I had to do `-aes256` instead of `-aes-256-cbc` – Brian Armstrong Jun 25 '14 at 17:37
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"That command can very effectively a strongly encrypt any file regardless of its size or format.", but oops on the last paragraph you acknowledge it can't encrypt large files at all.. perhaps you should not make such pompous claims of ultimate solution and all files when in the end your solution just doesn't work for large files. You should begin with the limitations and scope of your solution rather than finishing with it. – Timo Oct 09 '14 at 07:16
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Is there a bug report about that known issue with large >600MB files? Verifying the encrypted file is not possible if you only have the public key available. – Sampo Jul 15 '15 at 06:48
This is dangerous solution. It leads to LOSS of information for big files (>2GB). You need to split big file first – l0pan Jan 07 '17 at 22:55
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I was using this approach for creating database backups. However, once the backup size reached about 1.5GB, I was no longer able to decrypt the file. As mentioned above, there are bug reports about this issue. Use with caution. – Joe J Jul 07 '17 at 17:25
openssl has a limitation on not only the size of the input file but also the size of the output file, this is why this method does not work. But even if it did the AES algorithm cannot actually encrypt massively large files directly. This makes this answer moot. – Sammaye Aug 23 '17 at 14:52
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I wasn't able to make this working with **public** key (as asked in question). It requires a certificate. – Alleo Feb 01 '18 at 12:28
@Alleo, a self-signed certificate should be enough though and you can always make a self-signed certificate for a public+private keypair you generate. – Jan Hudec May 06 '21 at 08:43

score 47 · Answer 3 · answered Nov 27 '17 at 05:26

I found the instructions at http://www.czeskis.com/random/openssl-encrypt-file.html useful.

To paraphrase the linked site with filenames from your example:

Generate a symmetric key because you can encrypt large files with it
openssl rand -base64 32 > key.bin
Encrypt the large file using the symmetric key
openssl enc -aes-256-cbc -salt -in myLargeFile.xml \
  -out myLargeFile.xml.enc -pass file:./key.bin
Encrypt the symmetric key so you can safely send it to the other person
openssl rsautl -encrypt -inkey public.pem -pubin -in key.bin -out key.bin.enc
Destroy the un-encrypted symmetric key so nobody finds it
shred -u key.bin
At this point, you send the encrypted symmetric key (key.bin.enc) and the encrypted large file (myLargeFile.xml.enc) to the other person

The other person can then decrypt the symmetric key with their private key using
openssl rsautl -decrypt -inkey private.pem -in key.bin.enc -out key.bin
Now they can use the symmetric key to decrypt the file
openssl enc -d -aes-256-cbc -in myLargeFile.xml.enc \
  -out myLargeFile.xml -pass file:./key.bin

And you're done. The other person has the decrypted file and it was safely sent.

This is the best solution. Should have been top ranked and an accepted answer — King, Dec 14 '18 at 11:06

Hasturkun · Answer 4 · 2011-08-22T12:43:42.423

You can't directly encrypt a large file using rsautl. instead, do something like the following:

Generate a key using openssl rand, eg. openssl rand 32 -out keyfile
Encrypt the key file using openssl rsautl
Encrypt the data using openssl enc, using the generated key from step 1.
Package the encrypted key file with the encrypted data. the recipient will need to decrypt the key with their private key, then decrypt the data with the resulting key.

Ipswitch · Answer 5 · 2013-11-17T15:29:35.727

Encrypting a very large file using smime is not advised since you might be able to encrypt large files using the -stream option, but not decrypt the resulting file due to hardware limitations see: problem decrypting big files

As mentioned above Public-key crypto is not for encrypting arbitrarily long files. Therefore the following commands will generate a pass phrase, encrypt the file using symmetric encryption and then encrypt the pass phrase using the asymmetric (public key). Note: the smime includes the use of a primary public key and a backup key to encrypt the pass phrase. A backup public/private key pair would be prudent.

Random Password Generation

Set up the RANDFILE value to a file accessible by the current user, generate the passwd.txt file and clean up the settings

export OLD_RANDFILE=$RANDFILE
RANDFILE=~/rand1
openssl rand -base64 2048 > passwd.txt
rm ~/rand1
export RANDFILE=$OLD_RANDFILE

Encryption

Use the commands below to encrypt the file using the passwd.txt contents as the password and AES256 to a base64 (-a option) file. Encrypt the passwd.txt using asymetric encryption into the file XXLarge.crypt.pass using a primary public key and a backup key.

openssl enc -aes-256-cbc -a -salt -in XXLarge.data -out XXLarge.crypt -pass file:passwd.txt
openssl smime -encrypt -binary -in passwd.txt -out XXLarge.crypt.pass -aes256 PublicKey1.pem PublicBackupKey.pem
rm passwd.txt

Decryption

Decryption simply decrypts the XXLarge.crypt.pass to passwd.tmp, decrypts the XXLarge.crypt to XXLarge2.data, and deletes the passwd.tmp file.

openssl smime -decrypt -binary -in XXLarge.crypt.pass -out passwd.tmp -aes256 -recip PublicKey1.pem -inkey PublicKey1.key
openssl enc -d -aes-256-cbc -a -in XXLarge.crypt -out XXLarge2.data -pass file:passwd.tmp
rm passwd.tmp

This has been tested against >5GB files..

5365295400 Nov 17 10:07 XXLarge.data
7265504220 Nov 17 10:03 XXLarge.crypt
      5673 Nov 17 10:03 XXLarge.crypt.pass
5365295400 Nov 17 10:07 XXLarge2.data

Just my two cents: I would drop the `-a` in order to keep the encrypted file size smaller (no Base64 encoding) and add `-pbkdf2` for better key derivation (in 2021 openssl enc is complaining about simple `-pass`) — Ivin, May 03 '21 at 13:07
Isn't `smime` already doing the encrypt with symmetric cipher, then encrypt the key with RSA thing? Is the separate encryption just a workaround for the smime subcommand not working properly for large files? — Jan Hudec, May 06 '21 at 09:07
In your results and my tests, encrypted files are ~30% larger (original is already compressed). See XXLarge.crypt vs XXLarge.data. Is there any way to bring the size down with openssl parameters? — Marius, Apr 11 '23 at 06:26

Mohsenasm · Answer 6 · 2021-07-30T20:18:15.957

In more explanation for n. 'pronouns' m.'s answer,

Public-key crypto is not for encrypting arbitrarily long files. One uses a symmetric cipher (say AES) to do the normal encryption. Each time a new random symmetric key is generated, used, and then encrypted with the RSA cipher (public key). The ciphertext together with the encrypted symmetric key is transferred to the recipient. The recipient decrypts the symmetric key using his private key, and then uses the symmetric key to decrypt the message.

There is the flow of Encryption:

+---------------------+      +--------------------+
|                     |      |                    |
| generate random key |      |   the large file   |
|        (R)          |      |        (F)         |
|                     |      |                    |
+--------+--------+---+      +----------+---------+
         |        |                     |
         |        +------------------+  |
         |                           |  |
         v                           v  v
+--------+------------+     +--------+--+------------+
|                     |     |                        |
| encrypt (R) with    |     | encrypt (F)            |
| your RSA public key |     | with symmetric key (R) |
|                     |     |                        |
|  ASym(PublicKey, R) |     |     EF = Sym(F, R)     |
|                     |     |                        |
+----------+----------+     +------------+-----------+
           |                             |
           +------------+ +--------------+
                        | |
                        v v
         +--------------+-+---------------+
         |                                |
         |   send this files to the peer  |
         |                                |
         |     ASym(PublicKey, R) + EF    |
         |                                |
         +--------------------------------+

And the flow of Decryption:

   +----------------+        +--------------------+
   |                |        |                    |
   | EF = Sym(F, R) |        | ASym(PublicKey, R) |
   |                |        |                    |
   +-----+----------+        +---------+----------+
         |                             |
         |                             |
         |                             v
         |   +-------------------------+-----------------+
         |   |                                           |
         |   |             restore key (R)               |
         |   |                                           |
         |   | R <= ASym(PrivateKey, ASym(PublicKey, R)) |
         |   |                                           |
         |   +---------------------+---------------------+
         |                         |
         v                         v
     +---+-------------------------+---+
     |                                 |
     |       restore the file (F)      |
     |                                 |
     |      F <= Sym(Sym(F, R), R)     |
     |                                 |
     +---------------------------------+

Besides, you can use this commands:

# generate random symmetric key
openssl rand -base64 32 > /config/key.bin

# encryption
openssl rsautl -encrypt -pubin -inkey /config/public_key.pem -in /config/key.bin -out /config/key.bin.enc
openssl aes-256-cbc -a -pbkdf2 -salt -in  $file_name -out $file_name.enc -kfile /config/key.bin

# now you can send these files: $file_name.enc + /config/key.bin.enc

# decryption
openssl rsautl -decrypt -inkey /config/private_key.pem -in /config/key.bin.enc -out /config/key.bin
openssl aes-256-cbc -d -a -pbkdf2 -in $file_name.enc -out $file_name -kfile /config/key.bin

It is probably better to do `-kfile /config/key.bin` instead of `-k $(cat /config/key.bin)`, otherwise the passphrase would leak in the process list (task manager). — Yeti, Jul 03 '21 at 17:54

score 4 · Answer 7 · edited Aug 01 '16 at 12:35

To safely encrypt large files (>600MB) with openssl smime you'll have to split each file into small chunks:

# Splits large file into 500MB pieces
split -b 500M -d -a 4 INPUT_FILE_NAME input.part.

# Encrypts each piece
find -maxdepth 1 -type f -name 'input.part.*' | sort | xargs -I % openssl smime -encrypt -binary -aes-256-cbc -in % -out %.enc -outform DER PUBLIC_PEM_FILE

For the sake of information, here is how to decrypt and put all pieces together:

# Decrypts each piece
find -maxdepth 1 -type f -name 'input.part.*.enc' | sort | xargs -I % openssl smime -decrypt -in % -binary -inform DEM -inkey PRIVATE_PEM_FILE -out %.dec

# Puts all together again
find -maxdepth 1 -type f -name 'input.part.*.dec' | sort | xargs cat > RESTORED_FILE_NAME

score 2 · Answer 8 · edited May 23 '17 at 12:26

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Maybe you should check out the accepted answer to this (How to encrypt data in php using Public/Private keys?) question.

Instead of manually working around the message size limitation (or perhaps a trait) of RSA, it shows how to use the S/mime feature of OpenSSL to do the same thing and not needing to juggle with the symmetric key manually.

edited May 23 '17 at 12:26

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answered Aug 17 '12 at 01:20

Cray

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How to encrypt a large file in openssl using public key

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