Assuming you only modify the key schedule so that each of a 128-bit key is used at least once, would DES be about as secure as other ciphers such as AES? I am not talking about 2DES/3DES or other drawbacks like performance etc.

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    $\begingroup$ You should also define the new key schedule. When parameters changed, It usually requires new design and analysis. See AES-128 vs AES-256. $\endgroup$ – kelalaka Feb 18 at 17:08
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    $\begingroup$ One could take this question to be about Lucifer, or about Lucifer with the S-boxes as modified by the NSA to make them resist differential cryptanalysis. $\endgroup$ – Squeamish Ossifrage Feb 18 at 17:32

No, DES* (which I'll call your "DES modified to use 128 bit keys") would not be as secure as AES; two reasons spring immediately to mind:

  • Block size; DES* would still have 64 bit blocks; most block cipher modes start to leak information when you get close to the birthday bound; for DES*, that'd be 32Gigabytes, which isn't that long for common use. In contrast, AES (which has a 128 bit block size) has a birthday bound of circa 300Exabytes

  • Linear cryptanalysis; DES is known to be weak against linear cryptanalysis; depending on how you map the 128 bit keys to the DES* subkeys, DES* may very well be as well. Of course, AES is known to be immune to linear cryptanalysis

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    $\begingroup$ And that's already enough reason not to go for 128 bit DES, ignoring the less serious issues such as parity bits, weak keys and what-not. $\endgroup$ – Maarten Bodewes Feb 18 at 17:10

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