

I'm trying to comprehend the security impact of the different DH implementations on TLS. The main differences between the implementations are:
Where the ephemeral DH provides forward secrecy, thus provides additional security.
I'm not really sure how the elliptic curves impact the security of DH. I think I've previously read something like, the key size (or public certificate length?) of DH can be smaller, with the use of elliptic curves. So without the use of elliptic curves the key size should be at least 2048, to be considered secure.
Is my assumption correct? How can I identify the key size/ public certificate length (I'm not sure which is the correct term), to determine the security of DH in TLS?
Can I use the .pem file, used for DH, of my server to determine this? I've created the .pem file with the following command:
openssl dhparam out dhparam.pem 4096
and with the following command I believe I can determine the key length:
openssl dhparam inform PEM in ./dhparam.pem check text

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The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area) For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations...
If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’:
openssl pkey in xyz.key noout text
check the PrivateKey: (xxxx bit) in the output.
I'm trying to comprehend the security impact of the different DH implementations on TLS. The main differences between the implementations are:
Where the ephemeral DH provides forward secrecy, thus provides additional security.
I'm not really sure how the elliptic curves impact the security of DH. I think I've previously read something like, the key size (or public certificate length?) of DH can be smaller, with the use of elliptic curves. So without the use of elliptic curves the key size should be at least 2048, to be considered secure.
Is my assumption correct? How can I identify the key size/ public certificate length (I'm not sure which is the correct term), to determine the security of DH in TLS?
Can I use the .pem file, used for DH, of my server to determine this? I've created the .pem file with the following command:
openssl dhparam out dhparam.pem 4096
and with the following command I believe I can determine the key length:
openssl dhparam inform PEM in ./dhparam.pem check text
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Thanks for the reply. I'm still not sure I understand this correctly. So the length of modulus is the essential part, determining the security of the DH, right? With ECC, this is defined by the used curves. Without ECC, this is determined by the DH parameters (from the .pem file I mentioned). If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed. Is this correct? Thank you for your help, kind regards, SaAtomic  > Paul Yang paulyang.inf at gmail.com > Wed Jul 26 07:19:31 UTC 2017 > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area) > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations... > > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’: > > openssl pkey in xyz.key noout text > > check the PrivateKey: (xxxx bit) in the output.

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> On 26 Jul 2017, at 15:56, SaAtomic < [hidden email]> wrote:
>
> Thanks for the reply.
> I'm still not sure I understand this correctly.
>
> So the length of modulus is the essential part, determining the security of the DH, right?
Mostly.
> With ECC, this is defined by the used curves.
> Without ECC, this is determined by the DH parameters (from the .pem file I mentioned).
>
> If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed.
Yes, in that case, you only need an EC key (and also EC parameters to generate this key, of course)
>
> Is this correct?
>
> Thank you for your help,
> kind regards,
> SaAtomic
>
> 
> > Paul Yang paulyang.inf at gmail.com
> > Wed Jul 26 07:19:31 UTC 2017
> > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area)
> > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations...
> >
> > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’:
> >
> > openssl pkey in xyz.key noout text
> >
> > check the PrivateKey: (xxxx bit) in the output.

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The subject is much clearer to me now, thank you. The EC key you mentioned is not created manually, correct? This key is a result of ECC, which is done by OpenSSL. So if I set up a server offering TLS connections and only offer ECDH/ECDHE, no additional data has to be generated manually, correct? Kind regards, SaAtomic 26. Jul 2017 10:14 by [hidden email]: On 26 Jul 2017, at 15:56, SaAtomic <[hidden email]> wrote:
Thanks for the reply. I'm still not sure I understand this correctly.
So the length of modulus is the essential part, determining the security of the DH, right? Mostly.
With ECC, this is defined by the used curves. Without ECC, this is determined by the DH parameters (from the .pem file I mentioned).
If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed. Yes, in that case, you only need an EC key (and also EC parameters to generate this key, of course)
Is this correct?
Thank you for your help, kind regards, SaAtomic
 > Paul Yang paulyang.inf at gmail.com > Wed Jul 26 07:19:31 UTC 2017 > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area) > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations... > > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’: > > openssl pkey in xyz.key noout text > > check the PrivateKey: (xxxx bit) in the output.

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The subject is much clearer to me now, thank you.
The EC key you mentioned is not created manually, correct? This key is a result of ECC, which is done by OpenSSL.
So if I set up a server offering TLS connections and only offer ECDH/ECDHE, no additional data has to be generated manually, correct?
Ahh, that depends on how you use ECC in TLS. If you use something like ‘ECDHERSAXXXYYY’, you don’t need to manipulate the ECC stuff ‘manually’, since the authentication part of that cipher suite is RSA, just preparing an RSA key/certificate is enough. But if you want to use stuffs like ‘ECDHEECDSAXXXYYY’, you need to prepare ECC private key (and also the corresponding certificate that encodes the ECC public key). For ECDH ciphers (without the ‘E’ part), that’s more confused, IIRC, in such case the authentication part of that kind ciphers indicates what signature algorithm is used to sign the certificate, but not the ‘publickeytype’ in the certificate, so anyway you probably need to get a ECC key/certificate pair. But the without‘E’ version of such usage seems rare, I have never meet one in production environment...
And also, don’t mix up the ECC keys used in a `key exchange’ phase during TLS handshake with the keys used in ‘auth’ phase. I suggest you to read the following RFC documentation to get clear understandings on this: https://www.ietf.org/rfc/rfc4492.txt
Hope it helps : ) Kind regards, SaAtomic 26. Jul 2017 10:14 by [hidden email]: On 26 Jul 2017, at 15:56, SaAtomic <[hidden email]> wrote:
Thanks for the reply. I'm still not sure I understand this correctly.
So the length of modulus is the essential part, determining the security of the DH, right? Mostly.
With ECC, this is defined by the used curves. Without ECC, this is determined by the DH parameters (from the .pem file I mentioned).
If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed. Yes, in that case, you only need an EC key (and also EC parameters to generate this key, of course)
Is this correct?
Thank you for your help, kind regards, SaAtomic
> Paul Yang paulyang.inf at gmail.com> Wed Jul 26 07:19:31 UTC 2017 > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area) > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations... > > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’: > > openssl pkey in xyz.key noout text > > check the PrivateKey: (xxxx bit) in the output.

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Thank you for the elaboration and the link. One more followup question :) With
OpenVPN, when I configure a TLS cipher suite like
`TLSECDHEECDSAWITHAES128GCMSHA256`, I never manually created an
ECC private key. You mentioned that this is required for such cipher
suites. Does in this case OpenVPN take over that task, or is that yet
another special case? Kinds regards, SaAtomic 26. Jul 2017 10:39 by [hidden email]:
The subject is much clearer to me now, thank you.
The EC key you mentioned is not created manually, correct? This key is a result of ECC, which is done by OpenSSL.
So if I set up a server offering TLS connections and only offer ECDH/ECDHE, no additional data has to be generated manually, correct?
Ahh, that depends on how you use ECC in TLS. If you use something like ‘ECDHERSAXXXYYY’, you don’t need to manipulate the ECC stuff ‘manually’, since the authentication part of that cipher suite is RSA, just preparing an RSA key/certificate is enough. But if you want to use stuffs like ‘ECDHEECDSAXXXYYY’, you need to prepare ECC private key (and also the corresponding certificate that encodes the ECC public key). For ECDH ciphers (without the ‘E’ part), that’s more confused, IIRC, in such case the authentication part of that kind ciphers indicates what signature algorithm is used to sign the certificate, but not the ‘publickeytype’ in the certificate, so anyway you probably need to get a ECC key/certificate pair. But the without‘E’ version of such usage seems rare, I have never meet one in production environment...
And also, don’t mix up the ECC keys used in a `key exchange’ phase during TLS handshake with the keys used in ‘auth’ phase. I suggest you to read the following RFC documentation to get clear understandings on this: https://www.ietf.org/rfc/rfc4492.txt
Hope it helps : ) Kind regards, SaAtomic 26. Jul 2017 10:14 by [hidden email]: On 26 Jul 2017, at 15:56, SaAtomic <[hidden email]> wrote:
Thanks for the reply. I'm still not sure I understand this correctly.
So the length of modulus is the essential part, determining the security of the DH, right? Mostly.
With ECC, this is defined by the used curves. Without ECC, this is determined by the DH parameters (from the .pem file I mentioned).
If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed. Yes, in that case, you only need an EC key (and also EC parameters to generate this key, of course)
Is this correct?
Thank you for your help, kind regards, SaAtomic
> Paul Yang paulyang.inf at gmail.com> Wed Jul 26 07:19:31 UTC 2017 > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area) > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations... > > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’: > > openssl pkey in xyz.key noout text > > check the PrivateKey: (xxxx bit) in the output.

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No idea on OpenVPN, I guess you could ask them directly : )
Thank you for the elaboration and the link. One more followup question :) With
OpenVPN, when I configure a TLS cipher suite like
`TLSECDHEECDSAWITHAES128GCMSHA256`, I never manually created an
ECC private key. You mentioned that this is required for such cipher
suites. Does in this case OpenVPN take over that task, or is that yet
another special case? Kinds regards, SaAtomic 26. Jul 2017 10:39 by [hidden email]:
The subject is much clearer to me now, thank you.
The EC key you mentioned is not created manually, correct? This key is a result of ECC, which is done by OpenSSL.
So if I set up a server offering TLS connections and only offer ECDH/ECDHE, no additional data has to be generated manually, correct?
Ahh, that depends on how you use ECC in TLS. If you use something like ‘ECDHERSAXXXYYY’, you don’t need to manipulate the ECC stuff ‘manually’, since the authentication part of that cipher suite is RSA, just preparing an RSA key/certificate is enough. But if you want to use stuffs like ‘ECDHEECDSAXXXYYY’, you need to prepare ECC private key (and also the corresponding certificate that encodes the ECC public key). For ECDH ciphers (without the ‘E’ part), that’s more confused, IIRC, in such case the authentication part of that kind ciphers indicates what signature algorithm is used to sign the certificate, but not the ‘publickeytype’ in the certificate, so anyway you probably need to get a ECC key/certificate pair. But the without‘E’ version of such usage seems rare, I have never meet one in production environment...
And also, don’t mix up the ECC keys used in a `key exchange’ phase during TLS handshake with the keys used in ‘auth’ phase. I suggest you to read the following RFC documentation to get clear understandings on this: https://www.ietf.org/rfc/rfc4492.txt
Hope it helps : ) Kind regards, SaAtomic 26. Jul 2017 10:14 by [hidden email]: On 26 Jul 2017, at 15:56, SaAtomic <[hidden email]> wrote:
Thanks for the reply. I'm still not sure I understand this correctly.
So the length of modulus is the essential part, determining the security of the DH, right? Mostly.
With ECC, this is defined by the used curves. Without ECC, this is determined by the DH parameters (from the .pem file I mentioned).
If a server only supported ECDH or ECDHE, the DH parameters (.pem) file wouldn't even be needed. Yes, in that case, you only need an EC key (and also EC parameters to generate this key, of course)
Is this correct?
Thank you for your help, kind regards, SaAtomic
> Paul Yang paulyang.inf at gmail.com> Wed Jul 26 07:19:31 UTC 2017 > The ‘key size’ concept is usually referred to the length of modulus. (In public key crypto area) > For DH and ECDH, it (the size) ’s generated and defined in the ‘parameters’, as you pasted. Parameters are not exactly the final ‘keys’, they are the ‘materials’ to produce keys (both private ones and public ones), either for DH or ECDH. For DH, you generate parameters based on a given length of prime, and this length is what you called ‘key size’ (e.g. 2048), for ECC the parameters are generated based on named curves, such as prime192v1/prime239v1..., in this case, the ‘key > size’ is 192/239bit. In both case, the prime numbers are used as modulus being used while doing DH or EC crypto calculations... > > If you get either a DH or EC key, you could use the following command of OpenSSL to check the ‘key size’: > > openssl pkey in xyz.key noout text > > check the PrivateKey: (xxxx bit) in the output.

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